Internet Core Protocols/Oreilly-InternetCoreProtocols.iso/RFCs/rfc2642.txt

Text File | 1999-10-14 | 204.8 KB | 5,324 lines

Network Working Group                                            L. Kane
Request for Comments: 2642                Cabletron Systems Incorporated
Category: Informational                                      August 1999


                 Cabletron's VLS Protocol Specification

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   The Virtual LAN Link State Protocol (VLSP) is part of the InterSwitch
   Message Protocol (ISMP) which provides interswitch communication
   between switches running Cabletron's SecureFast VLAN (SFVLAN)
   product.  VLSP is used to determine and maintain a fully connected
   mesh topology graph of the switch fabric.  Each switch maintains an
   identical database describing the topology. Call-originating switches
   use the topology database to determine the path over which to route a
   call connection.

   VLSP provides support for equal-cost multipath routing, and
   recalculates routes quickly in the face of topological changes,
   utilizing a minimum of routing protocol traffic.

Table of Contents

    1. Introduction............................................  3
       1.1 Acknowledgments.....................................  3
       1.2 Data Conventions....................................  3
       1.3 ISMP Overview.......................................  4
    2. VLS Protocol Overview...................................  5
       2.1 Definitions of Commonly Used Terms..................  6
       2.2 Differences Between VLSP and OSPF...................  7
           2.2.1 Operation at the Physical Layer...............  8
           2.2.2 All Links Treated as Point-to-Point...........  8
           2.2.3 Routing Path Information......................  9
           2.2.4 Configurable Parameters.......................  9
           2.2.5 Features Not supported........................  9
       2.3 Functional Summary.................................. 10
       2.4 Protocol Packets.................................... 11



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       2.5 Protocol Data Structures............................ 12
       2.6 Basic Implementation Requirements................... 12
       2.7 Organization of the Remainder of This Document...... 13
    3. Interface Data Structure................................ 14
       3.1 Interface States.................................... 16
       3.2 Events Causing Interface State Changes.............. 18
       3.3 Interface State Machine............................. 21
    4. Neighbor Data Structure................................. 23
       4.1 Neighbor States..................................... 25
       4.2 Events Causing Neighbor State Changes............... 27
       4.3 Neighbor State Machine.............................. 29
    5. Area Data Structure..................................... 33
       5.1 Adding and Deleting Link State Advertisements....... 34
       5.2 Accessing Link State Advertisements................. 35
       5.3 Best Path Lookup.................................... 35
    6. Discovery Process....................................... 35
       6.1 Neighbor Discovery.................................. 36
       6.2 Bidirectional Communication......................... 37
       6.3 Designated Switch................................... 38
           6.3.1 Selecting the Designated Switch............... 39
       6.4 Adjacencies......................................... 41
    7. Synchronizing the Databases............................. 42
       7.1 Link State Advertisements........................... 43
           7.1.1 Determining Which
                 Link State Advertisement Is Newer............. 44
       7.2 Database Exchange Process........................... 44
           7.2.1 Database Description Packets.................. 44
           7.2.2 Negotiating the Master/Slave Relationship..... 45
           7.2.3 Exchanging Database Description Packets....... 46
       7.3 Updating the Database............................... 48
       7.4 An Example.......................................... 49
    8. Maintaining the Databases............................... 51
       8.1 Originating Link State Advertisements............... 52
           8.1.1 Switch Link Advertisements.................... 52
           8.1.2 Network Link Advertisements................... 55
       8.2 Distributing Link State Advertisements.............. 56
           8.2.1 Overview...................................... 57
           8.2.2 Processing an
                 Incoming Link State Update Packet............. 58
           8.2.3 Forwarding Link State Advertisements.......... 60
           8.2.4 Installing Link
                 State Advertisements in the Database.......... 62
           8.2.5 Retransmitting Link State Advertisements...... 63
           8.2.6 Acknowledging Link State Advertisements....... 64
       8.3 Aging the Link State Database....................... 66
           8.3.1 Premature Aging of Advertisements............. 66
    9. Calculating the Best Paths.............................. 67
   10. Protocol Packets........................................ 67



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       10.1 ISMP Packet Format................................. 68
            10.1.1 Frame Header................................ 69
            10.1.2 ISMP Packet Header.......................... 70
            10.1.3 ISMP Message Body........................... 71
       10.2 VLSP Packet Processing............................. 71
       10.3 Network Layer Address Information.................. 72
       10.4 VLSP Packet Header................................. 73
       10.5 Options Field...................................... 75
       10.6 Packet Formats..................................... 76
            10.6.1 Hello Packets............................... 76
            10.6.2 Database Description Packets................ 78
            10.6.3 Link State Request Packets.................. 80
            10.6.4 Link State Update Packets................... 82
            10.6.5 Link State Acknowledgment Packets........... 83
   11. Link State Advertisement Formats........................ 84
       11.1 Link State Advertisement Headers................... 84
       11.2 Switch Link Advertisements......................... 86
       11.3 Network Link Advertisements........................ 89
   12. Protocol Parameters..................................... 89
       12.1 Architectural Constants............................ 90
       12.2 Configurable Parameters............................ 91
   13. End Notes............................................... 93
   14. Security Considerations................................. 94
   15. References.............................................. 94
   16. Author's Address........................................ 94
   17. Full Copyright Statement................................ 95

1. Introduction

   This memo is being distributed to members of the Internet community
   in order to solicit reactions to the proposals contained herein.
   While the specification discussed here may not be directly relevant
   to the research problems of the Internet, it may be of interest to
   researchers and implementers.

1.1 Acknowledgments

   VLSP is derived from the OSPF link-state routing protocol described
   in [RFC2328], written by John Moy, formerly of Proteon, Inc.,
   Westborough, Massachusetts.  Much of the current memo has been drawn
   from [RFC2328].  Therefore, this author wishes to acknowledge the
   contribution Mr. Moy has (unknowingly) made to this document.

1.2 Data Conventions

   The methods used in this memo to describe and picture data adhere to
   the standards of Internet Protocol documentation [RFC1700].  In
   particular:



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      The convention in the documentation of Internet Protocols is to
      express numbers in decimal and to picture data in "big-endian"
      order.  That is, fields are described left to right, with the most
      significant octet on the left and the least significant octet on
      the right.  The order of transmission of the header and data
      described in this document is resolved to the octet level.
      Whenever a diagram shows a group of octets, the order of
      transmission of those octets is the normal order in which they are
      read in English.

      Whenever an octet represents a numeric quantity the left most bit
      in the diagram is the high order or most significant bit.  That
      is, the bit labeled 0 is the most significant bit.

      Similarly, whenever a multi-octet field represents a numeric
      quantity the left most bit of the whole field is the most
      significant bit.  When a multi-octet quantity is transmitted the
      most significant octet is transmitted first.

1.3 ISMP Overview

   The InterSwitch Message Protocol (ISMP) provides a consistent method
   of encapsulating and transmitting control messages exchanged between
   switches running Cabletron's SecureFast VLAN (SFVLAN) product, as
   described in [IDsfvlan].  ISMP provides the following services:

   o  Topology services.  Each switch maintains a distributed topology
      of the switch fabric by exchanging the following interswitch
      control messages with other switches:

   o  Interswitch Keepalive messages are sent by each switch to announce
      its existence to its neighboring switches and to establish the
      topology of the switch fabric.  (Interswitch Keepalive messages
      are exchanged in accordance with Cabletron's VlanHello protocol,
      described in [IDhello].)

   o  Interswitch Spanning Tree BPDU messages and Interswitch Remote
      Blocking messages are used to determine and maintain a loop-free
      flood path between all network switches in the fabric.  This flood

      path is used for all undirected interswitch messages -- that is,
      messages that are (potentially) sent to all switches in the switch
      fabric.

   o  Interswitch Link State messages (VLS protocol) are used to
      determine and maintain a fully connected mesh topology graph of
      the switch fabric.  Call-originating switches use the topology
      graph to determine the path over which to route a call connection.



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   o  Address resolution services.  Interswitch Resolve messages are
      used to resolve a packet destination address when the packet
      source and destination pair does not match a known connection.
      Interswitch New User messages are used to provide end-station
      address mobility between switches.

   o  Tag-based flooding.  A tag-based broadcast method is used to
      restrict the broadcast of unresolved packets to only those ports
      within the fabric that belong to the same VLAN as the source.

   o  Call tapping services.  Interswitch Tap messages are used to
      monitor traffic moving between two end stations.  Traffic can be
      monitored in one or both directions along the connection path.

   Note:  Previous versions of VLSP treated all links as if they were
   broadcast (multi-access).  Thus, if VLSP determines that a neighbor
   switch is running an older version of the protocol software (see
   Section 6.1), it will change the interface type to broadcast and
   begin exchanging Hello packets with the single neighbor switch.

2. VLS Protocol Overview

   VLSP is a dynamic routing protocol.  It quickly detects topological
   changes in the switch fabric (such as, switch interface failures) and
   calculates new loop-free routes after a period of convergence.  This
   period of convergence is short and involves a minimum of routing
   traffic.

   All switches in the fabric run the same algorithm and maintain
   identical databases describing the switch fabric topology.  This
   database contains each switch's local state, including its usable
   interfaces and reachable neighbors.  Each switch distributes its
   local state throughout the switch fabric by flooding.  From the
   topological database, each switch constructs a set of best path trees
   (using itself as the root) that specify routes to all other switches
   in the fabric.















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2.1 Definitions of Commonly Used Terms

   This section contains a collection of definitions for terms that have
   a specific meaning to the protocol and that are used throughout the
   text.

   Switch ID

      A 10-octet value that uniquely identifies the switch within the
      switch fabric.  The value consists of the 6-octet base MAC address
      of the switch, followed by 4 octets of zeroes.

   Network link

      The physical connection between two switches.  A link is
      associated with a switch interface.

      There are two physical types of network links supported by VLSP:

      o  Point-to-point links that join a single pair of switches.  A
         serial line is an example of a point-to-point network link.

      o  Multi-access broadcast links that support the attachment of
         multiple switches, along with the capability to address a
         single message to all the attached switches.  An attached
         ethernet is an example of a multi-access broadcast network
         link.

         A single topology can contain both types of links.  At startup,
         all links are assumed to be point-to-point.  A link is
         determined to be multi-access when more than one neighboring
         switch is discovered on the link.

   Interface

      The port over which a switch accesses one of its links.
      Interfaces are identified by their interface ID, a 10-octet value
      consisting of the 6-octet base MAC address of the switch, followed
      by the 4-octet local port number of the interface.

   Neighboring switches

      Two switches attached to a common link.








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   Adjacency

      A relationship formed between selected neighboring switches for
      the purpose of exchanging routing information.  Not every pair of
      neighboring switches become adjacent.

   Link state advertisement

      Describes the local state of a switch or a link.  Each link state
      advertisement is flooded throughout the switch fabric.  The
      collected link state advertisements of all switches and links form
      the protocol's topological database.

   Designated switch

      Each multi-access network link has a designated switch.  The
      designated switch generates a link state advertisement for the
      link and has other special responsibilities in the running of the
      protocol.

      The use of a designated switch permits a reduction in the number
      of adjacencies required on multi-access links.  This in turn
      reduces the amount of routing protocol traffic and the size of the
      topological database.

      The designated switch is selected during the discovery process.  A
      designated switch is not selected for a point-to-point network
      link.

   Backup designated switch

      Each multi-access network link has a backup designated switch.
      The backup designated switch maintains adjacencies with the same
      switches on the link as the designated switch.  This optimizes the
      failover time when the backup designated switch must take over for
      the (failed) designated switch.

      The backup designated switch is selected during the Discovery
      process.  A backup designated switch is not selected for a point-
      to-point network link.

2.2 Differences Between VLSP and OSPF

   The VLS protocol is derived from the OSPF link-state routing protocol
   described in [RFC2328].






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2.2.1 Operation at the Physical Layer

   The primary differences between the VLS and OSPF protocols stem from
   the fact that OSPF runs over the IP layer, while VLSP runs at the
   physical MAC layer.  This difference has the following repercussions:

   o  VLSP does not support features (such as fragmentation) that are
      typically provided by network layer service providers.

   o  Due to the unrelated nature of MAC address assignments, VLSP
      provides no summarization of the address space (such as, classical
      IP subnet information) or level 2 routing (such as,

      IS-IS Phase V DECnet).  Thus, VLSP does not support grouping
      switches into areas.  All switches exist in a single area.  Since
      a single domain exists within any switch fabric, there is no need
      for VLSP to provide interdomain reachability.

   o  As mentioned in Section 10.1.1, ISMP uses a single well-known
      multicast address for all packets.  However, parts of the VLS
      protocol (as derived from OSPF) are dependent on certain network
      layer addresses -- in particular, the AllSPFSwitches and
      AllDSwitches multicast addresses that drive the distribution of
      link state advertisements throughout the switch fabric.  In order
      to facilitate the implementation of the protocol at the physical
      MAC layer, network layer address information is encapsulated in
      the protocol packets (see Section 10.3).  This information is
      unbundled and packets are then processed as if they had been sent
      or received on that multicast address.

2.2.2 All Links Treated as Point-to-Point

   When the switch first comes on line, VLSP assumes all network links
   are point-to-point and no more than one neighboring switch will be
   discovered on any one port.  Therefore, at startup, VLSP does not
   send its own Hello packets over its network ports, but instead,
   relies on the VlanHello protocol [IDhello] for the discovery of its
   neighbor switches.  If a second neighbor is detected on a link, the
   link is then deemed multi-access and the interface type is changed to
   broadcast.  At that point, VLSP exchanges its own Hello packets with
   the switches on the link in order to select a designated switch and
   designated backup switch for the link.

   This method eliminates unnecessary duplication of message traffic and
   processing, thereby increasing the overall efficiency of the switch
   fabric.





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   Note:  Previous versions of VLSP treated all links as if they were
   broadcast (multi-access).  Thus, if VLSP determines that a neighbor
   switch is running an older version of the protocol software (see
   Section 6.1), it will change the interface type to broadcast and
   begin exchanging Hello packets with the single neighbor switch.

2.2.3 Routing Path Information

   Instead of providing the next hop to a destination, VLSP calculates
   and maintains complete end-to-end path information. On request, a
   list of individual port identifiers is generated describing a
   complete path from the source switch to the destination switch.  If
   multiple equal-cost routes exist to a destination switch, up to three
   paths are calculated and returned.

2.2.4 Configurable Parameters

   OSPF supports (and requires) configurable parameters.  In fact, even
   the default OSPF configuration requires that IP address assignments
   be specified.  On the other hand, no configuration information is
   ever required for the VLS protocol.  Switches are uniquely identified
   by their base MAC addresses and ports are uniquely identified by the
   base MAC address of the switch and a port number.

   While a developer is free to implement configurable parameters for
   the VLS protocol, the current version of VLSP supports configurable
   path metrics only.  Note that this has the following repercussions:

   o  All switches are assigned a switch priority of 1.  This forces the
      selection of the designated switch to be based solely on base MAC
      address.

   o  Authentication is not supported.

2.2.5 Features Not supported

   In addition to those features mentioned in the previous sections, the
   following OSPF features are not supported by the current version of
   VLSP:

   o  Periodic refresh of link state advertisements.  (This optimizes
      performance by eliminating unnecessary traffic between the
      switches.)

   o  Routing based on non-zero type of service (TOS).

   o  Use of external routing information for destinations outside the
      switch fabric.



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2.3 Functional Summary

   There are essentially four operational stages of the VLS protocol.

   o  Discovery Process The discovery process involves two steps:

      o  Neighboring switches are detected by the VlanHello protocol
         [IDhello] which then notifies VLSP of the neighbor.

      o  If more than one neighbor switch is detected on a single port,
         the link is determined to be multi-access.  VLSP then sends its
         own Hello packets over the link in order to discover the full
         set of neighbors on the link and select a designated switch and
         designated backup switch for the link.  Note that this
         selection process is unnecessary on point-to-point links.

      The discovery process is described in more detail in Section 6.

   o  Synchronizing the Databases

      Adjacencies are used to simplify and speed up the process of
      synchronizing the topological database (also known as the link
      state database) maintained by each switch in the fabric.  Each
      switch is only required to synchronize its database with those
      neighbors to which it is adjacent. This reduces the amount of
      routing protocol traffic across the fabric, particularly for
      multi-access links with multiple switches.

      The process of synchronizing the databases is described in more
      detail in Section 7.

   o  Maintaining the Databases

      Each switch advertises its state (also known as its link state)
      any time its link state changes.  Link state advertisements are
      distributed throughout the switch fabric using a reliable flooding
      algorithm that ensures that all switches in the fabric are
      notified of any link state changes.

      The process of maintaining the databases is described in more
      detail in Section 8.










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   o  Calculating the Best Paths

      The link state database consists of the collection of link state
      advertisements received from each switch.  Each switch uses its
      link state database to calculate a set of best paths, using itself
      as root, to all other switches in the fabric.

      The process of recalculating the set of best paths is described in
      more detail in Section 9.

2.4 Protocol Packets

   In addition to the frame header and the ISMP packet header described
   in Section 10.1, all VLS protocol packets share a common protocol
   header, described in Section 10.4.

   The VLSP packet types are listed below in Table 1.  Their formats are
   described in Section 10.6.

      Type   Packet Name            Protocol Function

      1      Hello                  Select DS and Backup DS
      2      Database Description   Summarize database contents
      3      Link State Request     Database download
      4      Link State Update      Database update
      5      Link State Ack         Flooding acknowledgment

                  Table 1: VLSP Packet Types

   The Hello packets are used to select the designated switch and the
   backup designated switch on multi-access links.  The Database
   Description and Link State Request packets are used to form
   adjacencies.  Link State Update and Link State Acknowledgment packets
   are used to update the topological database.

   Each Link State Update packet carries a set of link state
   advertisements.  A single Link State Update packet may contain the
   link state advertisements of several switches.  There are two
   different types of link state advertisement, as shown below in Table
   2.











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         LS     Advertisement    Advertisement Description
         Type   Name

         1      Switch link      Originated by all switches. This
                advertisements   advertisement describes the collected
                                 states of the switch's interfaces.

         2      Network link     Originated by the designated switch.
                advertisements   This advertisement contains the list
                                 of switches connected to the network
                                 link.

                  Table 2: VLSP Link State Advertisements

2.5 Protocol Data Structures

   The VLS protocol is described in this specification in terms of its
   operation on various protocol data structures.  Table 3 lists the
   primary VLSP data structures, along with the section in which they
   are described in detail.

         Structure Name                        Description

         Interface Data Structure              Section 3
         Neighbor Data Structure               Section 4
         Area Data Structure                   Section 5

                     Table 3: VLSP Data Structures

2.6 Basic Implementation Requirements

   An implementation of the VLS protocol requires the following pieces
   of system support:

   Timers

      Two types of timer are required.  The first type, known as a one-
      shot timer, expires once and triggers an event.  The second type,
      known as an interval timer, expires at preset intervals.  Interval
      timers are used to trigger events at periodic intervals.  The
      granularity of both types of timers is one second.

      Interval timers should be implemented in such a way as to avoid
      drift.  In some switch implementations, packet processing can
      affect timer execution.  For example, on a multi-access link with
      multiple switches, regular broadcasts can lead to undesirable
      synchronization of routing packets unless the interval timers have
      been implemented to avoid drift.  If it is not possible to



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      implement drift-free timers, small random amounts of time should
      be added to or subtracted from the timer interval at each firing.

   List manipulation primitives

      Much of the functionality of VLSP is described here in terms of
      its operation on lists of link state advertisements.  Any
      particular advertisement may be on many such lists. Implementation
      of VLSP must be able to manipulate these lists, adding and
      deleting constituent advertisements as necessary.

   Tasking support

      Certain procedures described in this specification invoke other
      procedures.  At times, these other procedures should be executed
      in-line -- that is, before the current procedure has finished.
      This is indicated in the text by instructions to "execute" a
      procedure.  At other times, the other procedures are to be
      executed only when the current procedure has finished.  This is
      indicated by instructions to "schedule" a task.  Implementation of
      VLSP must provide these two types of tasking support.

2.7 Organization of the Remainder of This Document

   The remainder of this document is organized as follows:

   o  Section 3 through Section 5 describe the primary data structures
      used by the protocol.  Note that this specification is presented
      in terms of these data structures in order to make explanations
      more precise.  Implementations of the protocol must support the
      functionality described, but need not use the exact data
      structures that appear in this specification.

   o  Section 6 through Section 9 describe the four operational stages
      of the protocol:  the discovery process, synchronizing the
      databases, maintaining the databases, and calculating the set of
      best paths.

   o  Section 10 describes the processing of VLSP packets and presents
      detailed descriptions of their formats.

   o  Section 11 presents detailed descriptions of link state
      advertisements.

   o  Section 12 summarizes the protocol parameters.






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3. Interface Data Structure

   The port over which a switch accesses a network link is known as the
   link interface.  Each switch maintains a separate interface data
   structure for each network link.

   The following data items are associated with each interface:

   Type

      The type of network to which the interface is attached -- point-
      to-point or broadcast (multi-access).  This data item is
      initialized to point-to-point when the interface becomes
      operational.  If a second neighbor is detected on the link after
      the first neighbor has been discovered, the link interface type is
      changed to broadcast.  The type remains as broadcast until the
      interface is declared down, at which time the type reverts to
      point-to-point.

   Note:  Previous versions of VLSP treated all links as if they were
   multi-access.  Thus, if VLSP determines that a neighbor switch is
   running an older version of the protocol software (see Section 6.1),
   it will change the interface type to broadcast.

   State

      The functional level of the interface.  The state of the interface
      is included in all switch link advertisements generated by the
      switch, and is also used to determine whether full adjacencies are
      allowed on the interface.  See Section 3.1 for a complete
      description of interface states.

   Interface identifier

      A 10-octet value that uniquely identifies the interface. This
      value consists of the 6-octet base MAC address of the neighbor
      switch, followed by the 4-octet local port number of the
      interface.

   Area ID

      A 4-octet value identifying the area.  Since VLSP does not support
      multiple areas, the value here is always zero.








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   HelloInterval

      The interval, in seconds, at which the switch sends VLSP Hello
      packets over the interface.  This parameter is not used on point-
      to-point links.

   SwitchDeadInterval

      The length of time, in seconds, that neighboring switches will
      wait before declaring the local switch dNeighboring switches

      A list of the neighboring switches attached to this network link.
      This list is created during the discovery process. Adjacencies are
      formed to one or more of these neighbors. The set of adjacent
      neighbors can be determined by examining the states of the
      neighboring switches as shown in their link state advertisements.

   Designated switch

      The designated switch selected for the multi-access network link.
      (A designated switch is not selected for a point-to-point link.)
      This data item is initialized to zero when the switch comes on-
      line, indicating that no designated switch has been chosen for the
      link.

   Backup designated switch

      The backup designated switch selected for the multi-access network
      link.  (A backup designated switch is not selected for a point-
      to-point link.)  This data item is initialized to zero when the
      switch comes on-line, indicating that no backup designated switch
      has been chosen for the link.

   Interface output cost(s)

      The cost of sending a packet over the interface.  The link cost is
      expressed in the link state metric and must be greater than zero.

   RxmtInterval

      The number of seconds between link state advertisement
      retransmissions, for adjacencies belonging to this interface. This
      value is also used to time the retransmission of Database
      Description and Link State Request packets.







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3.1 Interface States

   This section describes the various states of a switch interface. The
   states are listed in order of progressing functionality. For example,
   the inoperative state is listed first, followed by a list of the
   intermediate states through which the interface passes before
   attaining the final, fully functional state.  The specification makes
   use of this ordering by references such as "those interfaces in state
   greater than X".

   Figure 1 represents the interface state machine, showing the
   progression of interface state changes.  The arrows on the graph
   represent the events causing each state change.  These events are
   described in Section 3.2.  The interface state machine is described
   in detail in Section 3.3.

   Down

      This is the initial state of the interface.  In this state, the
      interface is unusable, and no protocol traffic is sent or received
      on the interface.  In this state, interface parameters are set to
      their initial values, all interface timers are disabled, and no
      adjacencies are associated with the interface.




























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       +-------+
       |  any  |  Interface   +----------+  Unloop Ind  +----------+
       | state | -----------> |   Down   | <----------- | Loopback |
       +-------+    Down      +----------+              +----------+
                                   |                         ^
                                   | Interface Up            |
           +-------+  [pt-to-pt]   |                         |
           | Point |<------------type?              Loop Ind |
           |  to   |               |                         |
           | Point |               | [broadcast]             |
           +-------+               V                     +-------+
                             +-----------+               |  any  |
                             |  Waiting  |               | state |
                             +-----------+               +-------+
                                   |
                       Backup Seen |
                                   | Wait Timer
                                   |
                                   |
      +----------+    Neighbor     V     Neighbor    +----------+
      |    DS    | <------------> [ ] <------------> | DS Other |
      +----------+     Change      ^      Change     +----------+
                                   |
                                   |
                   Neighbor Change |
                                   |
                                   V
                              +----------+
                              |  Backup  |
                              +----------+

                   Figure 1:  Interface State Machine


   Loopback

      In this state, the switch interface is looped back, either in
      hardware or in software.  The interface is unavailable for regular
      data traffic.

   Point-to-Point

      In this state, the interface is operational and is connected to a
      physical point-to-point link.  On entering this state, the switch
      attempts to form an adjacency with the neighboring switch.






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   Waiting

      In this state, the switch is attempting to identify the backup
      designated switch for the link by monitoring the Hello packets it
      receives.  The switch does not attempt to select a designated
      switch or a backup designated switch until it changes out of this
      state, thereby preventing unnecessary changes of the designated
      switch and its backup.

   DS Other

      In this state, the interface is operational and is connected to a
      multi-access broadcast link on which other switches have been
      selected as the designated switch and the backup designated
      switch.   On entering this state, the switch attempts to form
      adjacencies with both the designated switch and the backup
      designated switch.

   Backup

      In this state, the switch itself is the backup designated switch
      on the attached multi-access broadcast link.  It will be promoted
      to designated switch if the current designated switch fails.  The
      switch establishes adjacencies with all other switches attached to
      the link.  (See Section 6.3 for more information on the functions
      performed by the backup designated switch.)

   DS

      In this state, this switch itself is the designated switch on the
      attached multi-access broadcast link.  The switch establishes
      adjacencies with all other switches attached to the link.  The
      switch is responsible for originating network link advertisements
      for the link, containing link information for all switches
      attached to the link, including the designated switch itself.
      (See Section 6.3 for more information on the functions performed
      by the designated switch.)

3.2 Events Causing Interface State Changes

   The state of an interface changes due to an interface event.  This
   section describes these events.

   Interface events are shown as arrows in Figure 1, the graphic
   representation of the interface state machine.  For more information
   on the interface state machine, see Section 3.3.





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   Interface Up

      This event is generated by the VlanHello protocol [IDhello] when
      it discovers a neighbor switch on the interface.  The interface is
      now operational.  This event causes the interface to change out of
      the Down state.  The state it enters is determined by the
      interface type.  If the interface type is broadcast (multi-
      access), this event also causes the switch to begin sending
      periodic Hello packets out over the interface.

   Wait Timer

      This event is generated when the one-shot Wait timer expires,
      triggering the end of the required waiting period before the
      switch can begin the process of selecting a designated switch and
      a backup designated switch on a multi-access link.

   Backup Seen

      This event is generated when the switch has detected the existence
      or non-existence of a backup designated switch for the link, as
      determined in one of the following two ways:

      o  A Hello packet has been received from a neighbor that claims to
         be the backup designated switch.

      o  A Hello packet has been received from a neighbor that claims to
         be the designated switch.  In addition, the packet indicated
         that there is no backup.

   In either case, the interface must have bidirectional communication
   with its neighbor -- that is, the local switch must be listed in the
   neighbor's Hello packet.

   This event signals the end of the Waiting state.

   Neighbor change

      This event is generated when there has been one of the following
      changes in the set of bidirectional neighbors associated with the
      interface.  (See Section 4.1 for information on neighbor states.)

      o  Bidirectional communication has been established with a
         neighbor -- the state of the neighbor has changed to 2-Way or
         higher.

      o  Bidirectional communication with a neighbor has been lost --
         the state of the neighbor has changed to Init or lower.



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      o  A bidirectional neighbor has just declared itself to be either
         the designated switch or the backup designated switch, as
         detected by examination of that neighbor's Hello packets.

      o  A bidirectional neighbor is no longer declaring itself to be
         either the designated switch or the backup designated switch,
         as detected by examination of that neighbor's Hello packets.

      o  The advertised switch priority of a bidirectional neighbor has
         changed, as detected by examination of that neighbor's Hello
         packets.

      When this event occurs, the designated switch and the backup
      designated switch must be reselected.

      Loop Ind

         This event is generated when an interface enters the Loopback
         state.  This event can be generated by either the network
         management service or by the lower-level protocols.

      Unloop Ind

         This event is generated when an interface leaves the Loopback
         state.  This event can be generated by either the network
         management service or by the lower-level protocols.

      Interface Down

         This event is generated under the following two circumstances:

         o  The VlanHello [IDhello] protocol has determined that the
            interface is no longer functional.

         o  The neighbor state machine has detected a second neighboring
            switch on a link presumed to be of type point-to-point. In
            addition to generating the Interface Down event, the
            neighbor state machine changes the interface type to
            broadcast.

      In both instances, this event forces the interface state to Down.
      However, when the event is generated by the neighbor state
      machine, it is immediately followed by an Interface Up event.
      (See Section 4.3.)







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3.3 Interface State Machine

   This section presents a detailed description of the interface state
   machine.

   Interface states (see Section 3.1) change as the result of various
   events (see Section 3.2).  However, the effect of each event can
   vary, depending on the current state of the interface. For this
   reason, the state machine described in this section is organized
   according to the current interface state and the occurring event.
   For each state/event pair, the new interface state is listed, along
   with a description of the required processing.

   Note that when the state of an interface changes, it may be necessary
   to originate a new switch link advertisement.  See Section 8.1 for
   more information.

   Some of the processing described here includes generating events for
   the neighbor state machine.  For example, when an interface becomes
   inoperative, all neighbor connections associated with the interface
   must be destroyed.  For more information on the neighbor state
   machine, see Section 4.3.

   State(s):  Down
   Event:  Interface Up
   New state:  Depends on action routine
   Action:
      If the interface is a point-to-point link, set the interface state
      to Point-to-Point.  Otherwise, start the Hello interval timer,
      enabling the periodic sending of Hello packets over the interface.
      If the switch is not eligible to become the designated switch,
      change the interface state to DS Other. Otherwise, set the
      interface state to Waiting and start the one-shot wait timer.
      Create a new neighbor data structure for the neighbor switch,
      initialize all neighbor parameters and set the stateof the
      neighbor to Down.

   State(s):  Waiting
   Event:  Backup Seen
   New state:  Depends on action routine
   Action:
      Select the designated switch and backup designated switch for the
      attached link, as described in Section 6.3.1.  As a result of this
      selection, set the new state of the interface to either DS Other,
      Backup or DS.






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   State(s):  Waiting
   Event:  Wait Timer
   New state:  Depends on action routine
   Action:
      Select the designated switch and backup designated switch for the
      attached link, as described in Section 6.3.1.  As a result of this
      selection, set the new state of the interface to either DS Other,
      Backup or DS.

   State(s):  DS Other, Backup or DS
   Event:  Neighbor Change
   New state:  Depends on action routine
   Action:
      Reselect the designated switch and backup designated switch for
      the attached link, as described in Section 6.3.1.  As a result of
      this selection, set the new state of the interface to either DS
      Other, Backup or DS.

   State(s):  Any State
   Event:  Interface Down
   New state:  Down
   Action:
      Reset all variables in the interface data structure and disable
      all timers.  In addition, destroy all neighbor connections
      associated with the interface by generating the KillNbr event on
      all neighbors listed in the interface data structure.

   State(s):  Any State
   Event:  Loop Ind
   New state:  Loopback
   Action:
      Reset all variables in the interface data structure and disable
      all timers.  In addition, destroy all neighbor connections
      associated with the interface by generating the KillNbr event on
      all neighbors listed in the interface data structure.

   State(s):  Loopback
   Event:  Unloop Ind
   New state:  Down
   Action:
      No action is necessary beyond changing the interface state to Down
      because the interface was reset on entering the Loopback state.









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4. Neighbor Data Structure

   Each switch conducts a conversation with its neighboring switches and
   each conversation is described by a neighbor data structure.  A
   conversation is associated with a switch interface, and is identified
   by the neighboring switch ID.

   Note that if two switches have multiple attached links in common,
   multiple conversations ensue, each described by a unique neighbor
   data structure.  Each separate conversation is treated as a separate
   neighbor.

   The neighbor data structure contains all information relevant to any
   adjacency formed between the two neighbors.  Remember, however, that
   not all neighbors become adjacent.  An adjacency can be thought of as
   a highly developed conversation between two switches.

   State

      The functional level of the neighbor conversation.  See Section
      4.1 for a complete description of neighbor states.

   Inactivity timer

      A one-shot timer used to determine when to declare the neighbor
      down if no Hello packet is received from this (multi-access)
      neighbor.  The length of the timer is SwitchDeadInterval seconds,
      as contained in the neighbor's Hello packet.  This timer is not
      used on point-to-point links.

   Master/slave flag

      A flag indicating whether the local switch is to act as the master
      or the slave in the database exchange process (see Section 7.2).
      The master/slave relationship is negotiated when the conversation
      changes to the ExStart state.

   Sequence number

      A 4-octet number identifying individual Database Description
      packets. When the neighbor state ExStart is entered and the
      database exchange process is started, the sequence number is set
      to a value not previously seen by the neighboring switch. (One
      possible scheme is to use the switch's time of day counter.)  The
      sequence number is then incremented by the master with each new
      Database Description packet sent.  See Section 7.2 for more
      information on the database exchange process.




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   Neighbor ID

      The switch ID of the neighboring switch, as discovered by the
      VlanHello protocol [IDhello] or contained in the neighbor's Hello
      packets.

   Neighbor priority

      The switch priority of the neighboring switch, as contained in the
      neighbor's Hello packets.  Switch priorities are used when
      selecting the designated switch for the attached multi-access
      link.  Priority is not used on point-to-point links.

   Interface identifier

      A 10-octet value that uniquely identifies the interface over which
      this conversation is being held.  This value consists of the 6-
      octet base MAC address of the neighbor switch, followed by the 4-
      octet local port number of the interface.

   Neighbor's designated switch

      The switch ID identifying the neighbor's idea of the designated
      switch, as contained in the neighbor's Hello packets.  This value
      is used in the local selection of the designated switch.  It is
      not used on point-to-point links.

   Neighbor's backup designated switch

      The switch ID identifying the neighbor's idea of the backup
      designated switch, as contained in the neighbor's Hello packets.
      This value is used in the local selection of the backup designated
      switch.  It is not used on point-to-point links.

   Link state retransmission list

      The list of link state advertisements that have been forwarded
      over but not acknowledged on this adjacency.  The local switch
      retransmits these link state advertisements at periodic intervals
      until they are acknowledged or until the adjacency is destroyed.
      (For more information on retransmitting link state advertisements,
      see Section 8.2.5.)









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   Database summary list

      The set of link state advertisement headers that summarize the
      local link state database.  When the conversation changes to the
      Exchange state, this list is sent to the neighbor via Database
      Description packets.  (For more information on the synchronization
      of databases, see Section 7.)

   Link state request list

      The list of link state advertisements that must be received in
      order to synchronize with the neighbor switch's link state
      database.  This list is created as Database Description packets
      are received, and is then sent to the neighbor in Link State
      Request packets.  (For more information on the synchronization of
      databases, see Section 7.)

4.1 Neighbor States

   This section describes the various states of a conversation with a
   neighbor switch.  The states are listed in order of progressing
   functionality.  For example, the inoperative state is listed first,
   followed by a list of the intermediate states through which the
   conversation passes before attaining the final, fully functional
   state.  The specification makes use of this ordering by references
   such as "those neighbors/adjacencies in state greater than X".

   Figure 2 represents the neighbor state machine.  The arrows on the
   graph represent the events causing each state change.  These events
   are described in Section 4.2.  The neighbor state machine is
   described in detail in Section 4.3.

   Down

      This is the initial state of a neighbor conversation.

   Init

      In this state, the neighbor has been discovered, but bidirectional
      communication has not yet been established. All neighbors in this
      state or higher are listed in the VLS Hello packets sent by the
      local switch over the associated (multi-access) interface.









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          +----------+     KillNbr, LLDown,   +-----------+
          |   Down   | <--------------------- | any state |
          +----------+   or Inactivity Timer  +-----------+
               |
         Hello |
          Rcvd |
               |
               V
   +-----< [pt-to-pt?]
   | yes       |
   |           | no
   |           V
   |      +----------+   1-Way   +----------+
   |      |   Init   | <-------- | >= 2-way |
   |      +----------+           +----------+
   |           |
   |     2-Way |
   |      Rcvd |                  +-------+   AdjOK? +------------+
   |           +----------------> | 2-Way | <------- | >= ExStart |
   |           | (no adjacency)   +-------+     no   +------------+
   |           |
   |           V
   |      +---------+   Seq Number Mismatch  +-------------+
   +----> | ExStart | <--------------------- | >= Exchange |
          +---------+       or BadLSReq      +-------------+
               |
   Negotiation |
       Done    |
               V
          +----------+
          | Exchange |
          +----------+
               |
      Exchange |                        +--------+
        Done   +----------------------> |  Full  |
               | (request list empty)   +--------+
               |                             ^
               V                             |
          +---------+      Loading Done      |
          | Loading | ----------------------->
          +---------+

                  Figure 2: Neighbor State Machine








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   2-Way

      In this state, communication between the two switches is
      bidirectional.  This is the most advanced state short of beginning
      to establish an adjacency.  On a multi-access link, the designated
      switch and the backup designated switch are selected from the set
      of neighbors in state 2-Way or greater.

   ExStart

      This state indicates that the two switches have begun to establish
      an adjacency by determining which switch is the master, as well as
      the initial sequence number for Database Descriptor packets.
      Neighbor conversations in this state or greater are called
      adjacencies.

   Exchange

      In this state, the switches are exchanging Database Description
      packets.  (See Section 7.2 for a complete description of this
      process.)  All adjacencies in the Exchange state or greater are
      used by the distribution procedure (see Section 8.2), and are
      capable of transmitting and receiving all types of VLSP routing
      packets.

   Loading

      In this state, the local switch is sending Link State Request
      packets to the neighbor asking for the more recent advertisements
      that were discovered in the Exchange state.

   Full

      In this state, the two switches are fully adjacent.  These
      adjacencies will now appear in switch link and network link
      advertisements generated for the link.

4.2 Events Causing Neighbor State Changes

   The state of a neighbor conversation changes due to neighbor events.
   This section describes these events.

   Neighbor events are shown as arrows in Figure 2, the graphic
   representation of the neighbor state machine.  For more information
   on the neighbor state machine, see Section 4.3.






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   Hello Received

      This event is generated when a Hello packet has been received from
      a neighbor.

   2-Way Received

      This event is generated when the local switch sees its own switch
      ID listed in the neighbor's Hello packet, indicating that
      bidirectional communication has been established between the two
      switches.

   Negotiation Done

      This event is generated when the master/slave relationship has
      been successfully negotiated and initial packet sequence numbers
      have been exchanged.  This event signals the start of the database
      exchange process (see Section 7.2).

   Exchange Done

      This event is generated when the database exchange process is
      complete and both switches have successfully transmitted a full
      sequence of Database Description packets.  (For more information
      on the database exchange process, see Section 7.2.)

   BadLSReq

      This event is generated when a Link State Request has been
      received for a link state advertisement that is not contained in
      the database.  This event indicates an error in the
      synchronization process.

   Loading Done

      This event is generated when all Link State Updates have been
      received for all out-of-date portions of the database.  (See
      Section 7.3.)

   AdjOK?

      This event is generated when a decision must be made as to whether
      an adjacency will be established or maintained with the neighbor.
      This event will initiate some adjacencies and destroy others.







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   Seq Number Mismatch

      This event is generated when a Database Description packet has
      been received with any of the following conditions:

      o  The packet contains an unexpected sequence number.
      o  The packet (unexpectedly) has the Init bit set.
      o  The packet has a different Options field than was
         previously seen.

      These conditions all indicate that an error has occurred during
      the establishment of the adjacency.

   1-Way

      This event is generated when bidirectional communication with the
      neighbor has been lost.  That is, a Hello packet has been received
      from the neighbor in which the local switch is not listed.

   KillNbr

      This event is generated when further communication with the
      neighbor is impossible.

   Inactivity Timer

      This event is generated when the inactivity timer has expired,
      indicating that no Hello packets have been received from the
      neighbor in SwitchDeadInterval seconds.  This timer is used only
      on broadcast (multi-access) links.

   LLDown

      This event is generated by the lower-level switch discovery
      protocols and indicates that the neighbor is now unreachable.

4.3 Neighbor State Machine

   This section presents a detailed description of the neighbor state
   machine.

   Neighbor states (see Section 4.1) change as the result of various
   events (see Section 4.2).  However, the effect of each event can
   vary, depending on the current state of the conversation with the
   neighbor.  For this reason, the state machine described in this
   section is organized according to the current neighbor state and the
   occurring event.  For each state/event pair, the new neighbor state
   is listed, along with a description of the required processing.



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   Note that when the neighbor state changes as a result of an interface
   Neighbor Change event (see Section 3.2), it may be necessary to rerun
   the designated switch selection algorithm. In addition, if the
   interface associated with the neighbor conversation is in the DS
   state (that is, the local switch is the designated switch), changes
   in the neighbor state may cause a new network link advertisement to
   be originated (see Section 8.1).

   When the neighbor state machine must invoke the interface state
   machine, it is invoked as a scheduled task.  This simplifies
   processing, by ensuring that neither state machine executes
   recursively.

   State(s):  Down
   Event:  Hello Received
   New state:  Depends on the interface type
   Action:
      If the interface type of the associated link is point-to-point,
      change the neighbor state to ExStart.  Otherwise, change the
      neighbor state to Init and start the inactivity timer for the
      neighbor.  If the timer expires before another Hello packet is
      received, the neighbor switch is declared dead.

   State(s):  Init or greater
   Event:  Hello Received
   New state:  No state change
   Action:
      If the interface type of the associated link is point-to-point,
      determine whether this notification is for a different neighbor
      than the one previously seen. If so, generate an Interface Down
      event for the associated interface, reset the interface type to
      broadcast and generate an Interface Up event.

   Note:  This procedure of generating an Interface Down event and
   changing the interface type to broadcast is also executed if the
   neighbor for whom the notification was received is running an older
   version of the protocol software (see Section 6.1).  In previous
   versions of the protocol, all interfaces were treated as if they were
   broadcast.

      If the interface type is broadcast, reset the inactivity timer for
      the neighbor.









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   State(s):  Init
   Event:  2-Way Received
   New state:  Depends on action routine
   Action:
      Determine whether an adjacency will be formed with the neighbor
      (see Section 6.4).  If no adjacency is to be formed, change the
      neighbor state to 2-Way.

      Otherwise, change the neighbor state to ExStart.  Initialize the
      sequence number for this neighbor and declare the local switch to
      be master for the database exchange process.  (See Section 7.2.)

   State(s):  ExStart
   Event:  Negotiation Done
   New state:  Exchange
   Action:
      The Negotiation Done event signals the start of the database
      exchange process.  See Section 7.2 for a detailed description of
      this process.

   State(s):  Exchange
   Event:  Exchange Done
   New state:  Depends on action routine
   Action:
      If the neighbor Link state request list is empty, change the
      neighbor state to Full.  This is the adjacency's final state.

      Otherwise, change the neighbor state to Loading.  Begin sending
      Link State Request packets to the neighbor requesting the most
      recent link state advertisements, as discovered during the
      database exchange process.  (See Section 7.2.) These
      advertisements are listed in the link state request list
      associated with the neighbor.

   State(s):  Loading
   Event:  Loading Done
   New state:  Full
   Action:
      No action is required beyond changing the neighbor state to Full.
      This is the adjacency's final state.











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   State(s):  2-Way
   Event:  AdjOK?
   New state:  Depends on action routine
   Action:
      If no adjacency is to be formed with the neighboring switch (see
      Section 6.4), retain the neighbor state at 2-Way. Otherwise,
      change the neighbor state to ExStart.  Initialize the sequence
      number for this neighbor and declare the local switch to be master
      for the database exchange process.  (See Section 7.2.)

   State(s):  ExStart or greater
   Event:  AdjOK?
   New state:  Depends on action routine
   Action:
      If an adjacency should still be formed with the neighboring switch
      (see Section 6.4), no state change and no further action is
      necessary.  Otherwise, tear down the (possibly partially formed)
      adjacency.  Clear the link state retransmission list, database
      summary list and link state request list and change the neighbor
      state to 2-Way.

   State(s):  Exchange or greater
   Event:  Seq Number Mismatch
   New state:  ExStart
   Action:
      Tear down the (possibly partially formed) adjacency.  Clear the
      link state retransmission list, database summary list and link
      state request list.  Change the neighbor state to ExStart and make
      another attempt to establish the adjacency.

   State(s):  Exchange or greater
   Event:  BadLSReq
   New state:  ExStart
   Action:
      Tear down the (possibly partially formed) adjacency.  Clear the
      link state retransmission list, database summary list and link
      state request list.  Change the neighbor state to ExStart and make
      another attempt to establish the adjacency.

   State(s):  Any state
   Event:  KillNbr
   New state:  Down
   Action:
      Terminate the neighbor conversation.  Disable the inactivity timer
      and clear the link state retransmission list, database summary
      list and link state request list.





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   State(s):  Any state
   Event:  LLDown
   New state:  Down
   Action:
      Terminate the neighbor conversation.  Disable the inactivity timer
      and clear the link state retransmission list, database summary
      list and link state request list.

   State(s):  Any state
   Event:  Inactivity Timer
   New state:  Down
   Action:
      Terminate the neighbor conversation.  Disable the inactivity timer
      and clear the link state retransmission list, database summary
      list and link state request list.

   State(s):  2-Way or greater
   Event:  1-Way Received
   New state:  Init
   Action:
      Tear down the adjacency between the switches, if any.  Clear the
      link state retransmission list, database summary list and link
      state request list.

   State(s):  2-Way or greater
   Event:  2-Way received
   New state:  No state change
   Action:
      No action required.

   State(s):  Init
   Event:  1-Way received
   New state:  No state change
   Action:
            No action required.

5. Area Data Structure

   The area data structure contains all the information needed to run
   the basic routing algorithm.  One of its components is the link state
   database -- the collection of all switch link and network link
   advertisements generated by the switches.

   The area data structure contains the following items:







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   Area ID

      A 4-octet value identifying the area.  Since VLSP does not support
      multiple areas, the value here is always zero.

   Associated switch interfaces

      A list of interface IDs of the local switch interfaces connected
      to network links.

   Link state database

      The collection of all current link state advertisements for the
      switch fabric.  This collection consists of the following:

   Switch link advertisements

      A list of the switch link advertisements for all switches in the
      fabric.  Switch link advertisements describe the state of each
      switch's interfaces.

   Network link advertisements

      A list of the network link advertisements for all multi-access
      network links in the switch fabric.  Network link advertisements
      describe the set of switches currently connected to each link.

   Best path(s)

      A set of end-to-end hop descriptions for all equal-cost best paths
      from the local switch to every other switch in the fabric.  Each
      hop is specified by the interface ID of the next link in the path.
      Best paths are derived from the collected switch link and network
      link advertisements using the Dijkstra algorithm. [Perlman]

5.1 Adding and Deleting Link State Advertisements

   The link state database within the area data structure must contain,
   at most, a single instance of each link state advertisement.  To keep
   the database current, a switch adds link state advertisements to the
   database under the following conditions:

   o  When a link state advertisement is received during the
      distribution process

      o  When the switch itself generates a link state advertisement





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   (See Section 8.2.4 for information on installing link state
   advertisements.)

   Likewise, a switch deletes link state advertisements from the
   database under the following conditions:

   o  When a link state advertisement has been superseded by a newer
      instance during the flooding process

   o  When the switch generates a newer instance of one of its self-
      originated advertisements

   Note that when an advertisement is deleted from the link state
   database, it must also be removed from the link state retransmission
   list of all neighboring switches.

5.2 Accessing Link State Advertisements

   An implementation of the VLS protocol must provide access to
   individual link state advertisements, based on the advertisement's
   type, link state identifier, and advertising switch [1].  This lookup
   function is invoked during the link state distribution procedure and
   during calculation of the set of best paths.  In addition, a switch
   can use the function to determine whether it has originated a
   particular link state advertisement, and if so, with what sequence
   number.

5.3 Best Path Lookup

   An implementation of the VLS protocol must provide access to multiple
   equal-cost best paths, based on the base MAC addresses of the source
   and destination switches.  This lookup function should return up to
   three equal-cost paths.  Paths should be returned as lists of end-
   to-end hop information, with each hop specified as a interface ID of
   the next link in the path -- the 6-octet base MAC address of the next
   switch and the 4-octet local port number of the link interface.

6. Discovery Process

   The first operational stage of the VLS protocol is the discovery
   process.  During this stage, each switch dynamically detects its
   neighboring switches and establishes a relationship with each of
   these neighbors.  This process has the following component steps:








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   o  Neighboring switches are detected on each functioning network
      interface.

   o  Bidirectional communication is established with each neighbor
      switch.

   o  A designated switch and backup designated switch are selected for
      each multi-access network link.

   o  An adjacent relationship is established with selected neighbors on
      each link.

6.1 Neighbor Discovery

   When the switch first comes on line, VLSP assumes all network links
   are point-to-point and no more than one neighboring switch will be
   discovered on any one port.  Therefore, at startup, VLSP relies on
   the VlanHello protocol [IDhello] for the discovery of its neighbor
   switches.

   As each neighbor is detected, VlanHello triggers a Found Neighbor
   event, notifying VLSP that a new neighbor has been discovered.  (See
   [IDhello] for a description of the Found Neighbor event and the
   information passed.)  VLSP enters the neighbor switch ID in the list
   of known neighbors and creates a new neighbor data structure with a
   neighbor status of Down.  A Hello Received neighbor event is then
   generated, which changes the neighbor state to ExStart.

   There are two circumstances under which VLSP will change the type of
   an interface to broadcast:

   o  If VLSP receives a subsequent notification from VlanHello,
      specifying a second (different) neighbor switch on the port., the
      interface is then known to be multi-access.  VLSP generates an
      Interface Down event for the interface, resets the interface type
      to broadcast, and then generates an Interface Up event.

   o  If the functional level of the neighbor switch is less than 2, the
      neighbor is running a previous version of the VLSP software in
      which all links were treated as broadcast links. VLSP immediately
      changes the interface type to broadcast and generates an Interface
      Up event.

      In both cases, VLSP assumes control of communication over the
      interface by exchanging its own VLSP Hello packets with the
      neighbors on the link.





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   Note:  These Hello packets are in addition to the Interswitch
   Keepalive messages sent by VlanHello.  VlanHello still continues to
   monitor the condition of the interface and notifies VLSP of any
   change.

   Each Hello packet contains the following data used during the
   discovery process on multi-access links:

   o  The switch ID and priority of the sending switch

   o  Values specifying the interval timers to be used for sending Hello
      packets and deciding whether to declare a neighbor switch Down.

   o  The switch ID of the designated switch and the backup designated
      switch for the link, as understood by the sending switch

   o  A list of switch IDs of all neighboring switches seen so far on
      the link

   For a detailed description of the Hello packet format, see Section
   10.6.1.

   When VLSP receives a Hello packet (on a broadcast link), it first
   attempts to identify the sending switch by matching its switch ID to
   one of the known neighbors listed in the interface data structure.
   If this is the first Hello packet received from the switch, the
   switch ID is entered in the list of known neighbors and a new
   neighbor data structure is created with a neighbor status of Down.

   At this point, the remainder of the Hello packet is examined and the
   appropriate interface and neighbor events are generated.  In all
   cases, a neighbor Hello Received event is generated.  Other events
   may also be generated, triggering further steps in the discovery
   process or other actions, as appropriate.

   For a detailed description of the interface state machine, see
   Section 3.3.  For a detailed description of the neighbor state
   machine, see Section 4.3.

6.2 Bidirectional Communication

   Before a conversation can proceed with a neighbor switch,
   bidirectional communication must be established with that neighbor.
   Bidirectional communication is detected in one of two ways:







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   o  On a point-to-point link, the VlanHello protocol sees its own
      switch ID listed in an Interswitch Keepalive message it has
      received from the neighbor.

   o  On a multi-access link, VLSP sees its own switch ID listed in a
      VLSP Hello packet it has received from the neighbor.

   In either case, a neighbor 2-Way Received neighbor event is
   generated.

   Once bidirectional communication has been established with a
   neighbor, the local switch determines whether an adjacency will be
   formed with the neighbor.  However, if the link is a multi-access
   link, a designated switch and a backup designated switch must first
   be selected for the link.  The next section contains a description of
   the designated switch, the backup designated switch, and the
   selection process.

6.3 Designated Switch

   Every multi-access network link has a designated switch.  The
   designated switch performs the following functions for the routing
   protocol:

   o  The designated switch originates a network link advertisement on
      behalf of the link, listing the set of switches (including the
      designated switch itself) currently attached to the link. For a
      detailed description of network link advertisements, see Section
      11.3.

   o  The designated switch becomes adjacent to all other switches on
      the link.  Since the link state databases are synchronized across
      adjacencies, the designated switch plays a central part in the
      synchronization process.  For a description of the synchronization
      process, see Section 7.

   Each multi-access network link also has a backup designated switch.
   The primary function of the backup designated switch is to act as a
   standby for the designated switch.  If the current designated switch
   fails, the backup designated switch becomes the designated switch.

   To facilitate this transition, the backup designated switch forms an
   adjacency with every other switch on the link.  Thus, when the backup
   designated switch must take over for the designated switch, its link
   state database is already synchronized with the databases of all
   other switches on the link.





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   Note:  Point-to-point network links have neither a designated switch
   or a backup designated switch.

6.3.1 Selecting the Designated Switch

   When a multi-access link interface first becomes functional, the
   switch sets a one-shot Wait timer (with a value of SwitchDeadInterval
   seconds) for the interface.  The purpose of this timer is to ensure
   that all switches attached to the link have a chance to establish
   bidirectional communication before the designated switch and backup
   designated switch are selected for the link.

   When the Wait timer is set, the interface enters the Waiting state.
   During this state, the switch exchanges Hello packets with its
   neighbors attempting to establish bidirectional communication.  The
   interface leaves the Waiting state under one of the following
   conditions:

   o  The Wait timer expires.

   o  A Hello packet is received indicating that a designated switch or
      a backup designated switch has already been specified for the
      interface.

   At this point, if the switch sees that a designated switch has
   already been selected for the link, the switch accepts that
   designated switch, regardless of its own switch priority and MAC
   address.  This situation typically means the switch has come up late
   on a fully functioning link.  Although this makes it harder to
   predict the identity of the designated switch on a particular link,
   it ensures that the designated switch does not change needlessly,
   necessitating a resynchronization of the databases.

   If no designated switch is currently specified for the link, the
   switch begins the actual selection process.  Note that this selection
   algorithm operates only on a list of neighbor switches that are
   eligible to become the designated switch.  A neighbor is eligible to
   be the designated switch if it has a switch priority greater than
   zero and its neighbor state is 2-Way or greater.  The local switch
   includes itself on the list of eligible switches as long as it has a
   switch priority greater than zero.

   The selection process includes the following steps:

   1. The current values of the link's designated switch and backup
      designated switch are saved for use in step 6.

   2. The new backup designated switch is selected as follows:



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      a) Eliminate from consideration those switches that have declared
         themselves to be the designated switch.

      b) If one or more of the remaining switches have declared
         themselves to be the backup designated switch, eliminate from
         consideration all other switches.

      c) From the remaining list of eligible switches, select the switch
         having the highest switch priority as the backup designated
         switch.  If multiple switches have the same (highest) priority,
         select the switch with the highest switch ID as the backup
         designated switch.

   3. The new designated switch is selected as follows:

      a) If one or more of the switches have declared themselves to be
         the designated switch, eliminate from consideration all other
         switches.

      b) From the remaining list of eligible switches, select the switch
         having the highest switch priority as the designated switch.
         If multiple switches have the same (highest) priority, select
         the switch with the highest switch ID as the designated switch.

   4. If the local switch has been newly selected as either the
      designated switch or the backup designated switch, or is now no
      longer the designated switch or the backup designated switch,
      repeat steps 2 and 3, above, and then proceed to step 5.

      If the local switch is now the designated switch, it will
      eliminate itself from consideration at step 2a when the selection
      of the backup designated switch is repeated. Likewise, if the
      local switch is now the backup designated switch, it will
      eliminate itself from consideration at step 3a when the selection
      of the designated switch is repeated. This ensures that no switch
      will select itself as both backup designated switch and designated
      switch [2].

   5. Set the interface state to the appropriate value, as follows:

   o  If the local switch is now the designated switch, set the
      interface state to DS.

   o  If the local switch is now the backup designated switch, set the
      interface state to Backup.

   o  Otherwise, set the interface state to DS Other.




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   6. If either the designated switch or backup designated switch has
      now changed, the set of adjacencies associated with this link must
      be modified.  Some adjacencies may need to be formed, while others
      may need to be broken.  Generate the neighbor AdjOK? event for all
      neighbors with a state of 2-Way or higher to trigger a
      reexamination of adjacency eligibility.

   Caution:  If VLSP is implemented with configurable parameters, care
   must be exercised in specifying the switch priorities.  Note that if
   the local switch is not itself eligible to become the designated
   switch (i.e., it has a switch priority of 0), it is possible that
   neither a backup designated switch nor a designated switch will be
   selected by the above procedure.  Note also that if the local switch
   is the only attached switch that is eligible to become the designated
   switch, it will select itself as designated switch and there will be
   no backup designated switch for the link.  For this reason, it is
   advisable to specify a default switch priority of 1 for all switches.

6.4 Adjacencies

   VLSP creates adjacencies between neighboring switches for the purpose
   of exchanging routing information.  Not every two neighboring
   switches will become adjacent.  On a multi-access link, an adjacency
   is only formed between two switches if one of them is either the
   designated switch or the backup designated switch.

   Note that an adjacency is bound to the network link that the two
   switches have in common.  Therefore, if two switches have multiple
   links in common, they may also have multiple adjacencies between
   them.

   The decision to form an adjacency occurs in two places in the
   neighbor state machine:

   o  When bidirectional communication is initially established with the
      neighbor.

   o  When the designated switch  or backup designated switch on the
      attached link changes.

   The rules for establishing an adjacency between two neighboring
   switches are as follows:

   o  On a point-to-point link, the two neighboring switches always
      establish an adjacency.

   o  On a multi-access link, an adjacency is established with the
      neighboring switch under one of the following conditions:



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      o  The local switch itself is the designated switch.
      o  The local switch itself is the backup designated switch.
      o  The neighboring switch is the designated switch.
      o  The neighboring switch is the backup designated switch.

   If no adjacency is formed between two neighboring switches, the state
   of the neighbor conversation remains set to 2-Way.

7. Synchronizing the Databases

   In an SPF-based routing algorithm, it is important for the link state
   databases of all switches to stay synchronized.  VLSP simplifies this
   process by requiring only adjacent switches to remain synchronized.

   The synchronization process begins when the switches attempt to bring
   up the adjacency.  Each switch in the adjacency describes its
   database by sending a sequence of Database Description packets to its
   neighbor.  Each Database Description packet describes a set of link
   state advertisements belonging to the database.  When the neighbor
   sees a link state advertisement that is more recent than its own
   database copy, it makes a note to request this newer advertisement.

   During this exchange of Database Description packets (known as the
   database exchange process), the two switches form a master/slave
   relationship.  Database Description packets sent by the master are
   known as polls, and each poll contains a sequence number.  Polls are
   acknowledged by the slave by echoing the sequence number in the
   Database Description response packet.

   When all Database Description packets have been sent and
   acknowledged, the database exchange process is completed.  At this
   point, each switch in the exchange has a list of link state
   advertisements for which its neighbor has more recent instances.
   These advertisements are requested using Link State Request packets.

   Once the database exchange process has completed and all Link State
   Requests have been satisfied, the databases are deemed synchronized
   and the neighbor states of the two switches are set to Full,
   indicating that the adjacency is fully functional. Fully functional
   adjacencies are advertised in the link state advertisements of the
   two switches [3].










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7.1 Link State Advertisements

   Link state advertisements form the core of the database from which a
   switch calculates the set of best paths to the other switches in the
   fabric.

   Each link state advertisement begins with a standard header. This
   header contains three data items that uniquely identify the link
   state advertisement.

   o  The link state type.  Possible values are as follows:

      1   Switch link advertisement -- describes the collected states of
         the switch's interfaces.

      2   Network link advertisement -- describes the set of switches
         attached to the network link.

   o  The link state ID, defined as follows:

      o  For a switch link advertisement -- the switch ID of the
         originating switch

      o  For a network link advertisement -- the switch ID of the
         designated switch for the link

   o  The switch ID of the advertising switch -- the switch that
      generated the advertisement

   The link state advertisement header also contains three data items
   that are used to determine which instance of a particular link state
   advertisement is the most current.  (See Section 7.1.1 for a
   description of how to determine which instance of a link state
   advertisement is the most current.)

   o  The link state sequence number

   o  The link state age, stored in seconds

   o  The link state checksum, a 16-bit unsigned value calculated for
      the entire contents of the link state advertisement, with the
      exception of the age field

   The remainder of each link state advertisement contains data specific
   to the type of the advertisement.  See Section 11 for a detailed
   description of the link state header, as well as the format of a
   switch link or network link advertisement.




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7.1.1 Determining Which Link State Advertisement Is Newer

   At various times while synchronizing or updating the link state
   database, a switch must determine which instance of a particular link
   state advertisement is the most current.  This decision is made as
   follows:

   o  The advertisement having the greater sequence number is the most
      current.

   o  If both instances have the same sequence number, then:

      o  If the two instances have different checksum values, then the
         instance having the larger checksum is considered the most
         current [4].

   o  If both instances have the same sequence number and the same
      checksum value, then:

      o  If one (and only one) of the instances is of age MaxAge, then
         the instance of age MaxAge is considered the most current [5].

      o  Else, if the ages of the two instances differ by more than
         MaxAgeDiff, the instance having the smaller (younger) age is
         considered the most current [6].

      o  Else, the two instances are considered identical.

7.2 Database Exchange Process

   There are two stages to the database exchange process:

   o  Negotiating the master/slave relationship
   o  Exchanging database summary information

7.2.1 Database Description Packets

   Database Description packets are used to describe a switch's link
   state database during the database exchange process.  Each Database
   Description packet contains a list of headers of the link state
   advertisements currently stored in the sending switch's database.
   (See Section 11.1 for a description of a link state advertisement
   header.)

   In addition to the link state headers, each Database Description
   packet contains the following data items:





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   o  A flag (the M-bit) indicating whether or not more packets are to
      follow.  Depending on the size of the local database and the
      maximum size of the packet, the list of headers in any particular
      Database Description packet may be only a partial list of the
      total database.  When the M-bit is set, the list of headers is
      only a partial list and more headers are to follow in subsequent
      packets.

   o  A flag (the I-bit) indicating whether or not this is the first
      Database Description packet sent for this execution of the
      database exchange process.

   o  A flag (the MS-bit) indicating whether the sending switch thinks
      it is the master or the slave in the database exchange process.
      If the flag is set, the switch thinks it is the master.

   o  A 4-octet sequence number for the packet.

   While the switches are negotiating the master/slave relationship,
   they exchange "empty" Database Description packets.  That is, packets
   that contain no link summary information.  Instead, the flags and
   sequence number constitute the information required for the
   negotiation process.

   See Section 10.6.2 for a more detailed description of a Database
   Description packet.

7.2.2 Negotiating the Master/Slave Relationship

   Before two switches can begin the actual exchange of database
   information, they must decide between themselves who will be the
   master in the exchange process and who will be the slave.  They must
   also agree on the starting sequence number for the Database
   Description packets.

   Once a switch has decided to form an adjacency with a neighboring
   switch, it sets the neighbor state to ExStart and begins sending
   empty Database Description packets to its neighbor.  These packets
   contain the starting sequence number the switch plans to use in the
   exchange process.  Also, the I-bit and M-bit flags are set, as well
   as the MS-bit.  Thus, each switch in the exchange begins by believing
   it will be the master.

   Empty Database Description packets are retransmitted every
   RxmtInterval seconds until the neighbor responds.






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   When a switch receives an empty Database Description packet from its
   neighbor, it determines which switch will be the master by comparing
   the switch IDs.  The switch with the highest switch ID becomes the
   master of the exchange.  Based on this determination, the switch
   proceeds as follows:

   o  If the switch is to be the slave of the database exchange process,
      it acknowledges that it is the slave by sending another empty
      Database Description packet to the master. This packet contains
      the master's sequence number and has the MS-bit and the I-bit
      cleared.

   o  The switch then generates a neighbor event of Negotiation Done to
      change its neighbor state to Exchange and waits for the first
      non-empty Database Description packet from the master.

   o  If the switch is to be the master of the database exchange, it
      waits to receive an acknowledgment from its neighbor -- that is,
      an empty Database Description packet with the MS-bit and I-bit
      cleared and containing the sequence number it (the master)
      previously sent.

   o  When it receives the acknowledgment, it generates a neighbor event
      of Negotiation Done to change its neighbor state to Exchange and
      begin the actual exchange of Database Description packets.

   Note that during the negotiation process, the receipt of an
   inconsistent packet will result in a neighbor event of Seq Number
   Mismatch, terminating the process.  See Section 4.3 for more
   information.

7.2.3 Exchanging Database Description Packets

   Once the neighbor state changes to Exchange, the switches begin the
   exchange of Database Description packets containing link state
   summary data.  The process proceeds as follows:

   1. The master sends a packet containing a list of link state headers.
      If the packet contains only a portion of the unexchanged database
      -- that is, more Database Description packets are to follow -- the
      packet has the M-bit set.  The MS-bit is set and the I-bit is
      clear.

      If the slave does not acknowledge the packet within RxmtInterval
      seconds, the master retransmits the packet.






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   2. When the slave receives a packet, it first checks the sequence
      number to see if the packet is a duplicate.  If so, it simply
      acknowledges the packet by clearing the MS-bit and returning the
      packet to the master.  (Note that the slave acknowledges all
      Database Description packets that it receives, even those that are
      duplicates.)

      Otherwise, the slave processes the packet by doing the following:

      o  For each link state header listed in the packet, the slave
         searches its own link state database to determine whether it
         has an instance of the advertisement.

      o  If the slave does not have an instance of the link state
         advertisement, or if the instance it does have is older than
         the instance listed in the packet, it creates an entry in its
         link state request list in the neighbor data structure.  See
         Section 7.1.1 for a description of how to determine which
         instance of a link state advertisement is the newest.

      o  When the slave has examined all headers, it acknowledges the
         packet by turning the MS-bit off and returning the packet to
         the master.


   3. When the master receives the first acknowledgment for a particular
      Database Description packet, it processes the acknowledgment as
      follows:

      o  For each link state header listed in the packet, the master
         checks to see if the slave has indicated it has an instance of
         the link state advertisement that is newer than the instance
         the master has in its own database.  If so, the master creates
         an entry in its link state request list in the neighbor data
         structure.

      o  The master then increments the sequence number and sends
         another packet containing the next set of link state summary
         information, if any.

      Subsequent acknowledgments for the Database Description packet
      (those with the same sequence number) are discarded.

      When the master sends the last portion of its database summary
      information, it clears the M-bit in the packet to indicate that no
      more packets are to be sent.





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   4. When the slave receives a Database Description packet with the M-
      bit clear, it processes the packet, as described above in step 2.
      After it has completed processing and has acknowledged the packet
      to the master, it generates an Exchange Done neighbor event and
      its neighbor state changes to Loading.

      The database exchange process is now complete for the slave, and
      it begins the process of requesting those link state
      advertisements for which the master has more current instances
      (see Section 7.3).

   5. When the master receives an acknowledgment for the final Database
      Description packet, it processes the acknowledgment as described
      above in step 3.  Then it generates an Exchange Done neighbor
      event and its neighbor state changes to Loading.

      The database exchange process is now complete for the master, and
      it begins the process of requesting those link state
      advertisements for which the slave has more current instances (see
      Section 7.3).

   Note that during this exchange, the receipt of an inconsistent packet
   will result in a neighbor event of Seq Number Mismatch, terminating
   the process.  See Section 4.3 for more information.

7.3 Updating the Database

   When either switch completes the database exchange process and its
   neighbor state changes to Loading, it has a list of link state
   advertisements for which the neighboring switch has a more recent
   instance.  This list is stored in the neighbor data structure as the
   link state request list.

   To complete the synchronization of its database with that of its
   neighbor, the switch must obtain the most current instances of those
   link state advertisements.

   The switch requests these advertisements by sending its neighbor a
   Link State Request packet containing the description of one or more
   link state advertisement, as defined by the advertisement's type,
   link state ID, and advertising switch.  (For a detailed description
   of the Link State Request packet, see Section 10.6.3.)  The switch
   continues to retransmit this packet every RxmtInterval seconds until
   it receives a reply from the neighbor.







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   When the neighbor switch receives the Link State Request packet, it
   responds with a Link State Update packet containing its most current
   instance of each of the requested advertisements.  (Note that the
   neighboring switch can be in any of the Exchange, Loading or Full
   neighbor states when it responds to a Link State Request packet.)

   If the neighbor cannot locate a particular link state advertisement
   in its database, something has gone wrong with the synchronization
   process.  The switch generates a BadLSReq neighbor event and the
   partially formed adjacency is torn down. See Section 4.3 for more
   information.

   Depending on the size of the link state request list, it may take
   more than one Link State Request packet to obtain all the necessary
   advertisements.  Note, however, that there must at most one Link
   State Request packet outstanding at any one time.

7.4 An Example

   Figure 3 shows an example of an adjacency being formed between two
   switches -- S1 and S2 -- connected to a network link.  S2 is the
   designated switch for the link and has a higher switch ID than S1.

   The neighbor state changes that each switch goes through are listed
   on the sides of the figure.


























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   +--------+                                     +--------+
   | Switch |                                     | Switch |
   |   S1   |                                     |   S2   |
   +--------+                                     +--------+
      Down                                           Down
                     Hello (DS=0, seen=0)
            ------------------------------------->
                                                     Init
                  Hello (DS=S2, seen=...,S1)
            <-------------------------------------
   ExStart
             DB Description (Seq=x, I, M, Master)
            ------------------------------------->
                                                     ExStart
             DB Description (Seq=y, I, M, Master)
            <-------------------------------------
   xchange
               DB Description (Seq=y, M, Slave)
            ------------------------------------->
                                                     Exchange
             DB Description (Seq=y+1, M, Master)
            <-------------------------------------
              DB Description (Seq=y+1, M, Slave)
            ------------------------------------->
                              .
                              .
                              .

               DB Description (Seq=y+n, Master)
            <-------------------------------------
                DB Description (Seq=y+n, Slave)
            ------------------------------------->
   Loading                                           Full
                       Link State Request
            <-------------------------------------
                       Link State Update
            ------------------------------------->
                              .
                              .
                              .

                       Link State Request
            <-------------------------------------
                       Link State Update
            ------------------------------------->
    Full

         Figure 3: An Example of Bringing Up an Adjacency



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   At the top of Figure 3, S1's interface to the link becomes
   operational, and S1 begins sending Hello packets over the interface.
   At this point, S1 does not yet know the identity of the designated
   switch or of any other neighboring switches.  S2 receives the Hello
   packet from S1 and changes its neighbor state to Init.  In its next
   Hello packet, S2 indicates that it is itself the designated switch
   and that it has received a Hello packet from S1.  S1 receives the
   Hello packet and changes its state to ExStart, starting the process
   of bringing up the adjacency.

   S1 begins by asserting itself as the master.  When it sees that S2 is
   indeed the master (because of S2's higher switch ID), S1 changes to
   slave and adopts S2's sequence number.  Database Description packets
   are then exchanged, with polls coming from the master (S2) and
   acknowledgments from the slave (S1).  This sequence of Database
   Description packets ends when both the poll and associated
   acknowledgment have the M-bit off.

   In this example, it is assumed that S2 has a completely up-to-date
   database and immediately changes to the Full state. S1 will change to
   the Full state after updating its database by sending Link State
   Request packets and receiving Link State Update packets in response.

   Note that in this example, S1 has waited until all Database
   Description packets have been received from S2 before sending any
   Link State Request packets.  However, this need not be the case.  S1
   could interleave the sending of Link State Request packets with the
   reception of Database Description packets.

8. Maintaining the Databases

   Each switch advertises its state (also known as its link state) by
   originating switch link advertisements.  In addition, the designated
   switch on each network link advertises the state of the link by
   originating network link advertisements.

   As described in Section 7.1, link state advertisements are uniquely
   identified by their type, link state ID, and advertising switch.

   Link state advertisements are distributed throughout the switch
   fabric using a reliable flooding algorithm that ensures that all
   switches in the fabric are notified of any link state changes.







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8.1 Originating Link State Advertisements

   A new instance of each link state advertisement is originated any
   time the state of the switch or link changes.  When a new instance of
   a link state advertisement is originated, its sequence number is
   incremented, its age is set to zero, and its checksum is calculated.
   The advertisement is then installed into the local link state
   database and forwarded out all fully operational interfaces (that is,
   those interfaces with a state greater than Waiting) for distribution
   throughout the switch fabric.  See Section 8.2.4 for a description of
   the installation of the advertisement into the link state database
   and Section 8.2.5 for a description of how advertisements are
   forwarded.

   A switch originates a new instance of a link state advertisement as a
   result of the following events:

   o  The state of one of the switch's interfaces changes such that the
      contents of the associated switch link advertisement changes.

   o  The designated switch on any of the switch's attached network
      links changes.  The switch originates a new switch link
      advertisement.  Also, if the switch itself is now the designated
      switch, it originates a new network link advertisement for the
      link.

   o  One of the switch's neighbor states changes to or from Full. If
      this changes the contents of the associated switch link
      advertisement, a new instance is generated.  Also, if the switch
      is the designated switch for the attached network link, it
      originates a new network link advertisement for the link.

   Two instances of the same link state advertisement must not be
   originated within the time period MinLSInterval.  Note that this may
   require that the generation of the second instance to be delayed up
   to MinLSInterval seconds.

8.1.1 Switch Link Advertisements

   A switch link advertisement describes the collected states of all
   functioning links attached to the originating switch -- that is, all
   attached links with an interface state greater than Down.  A switch
   originates an empty switch link advertisement when it first becomes
   functional.  It then generates a new instance of the advertisement
   each time one of its interfaces reaches a fully functioning state
   (Point-to-Point or better).





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   Each link in the advertisement is assigned a type, based on the state
   of interface, as shown in Table 4.

            Interface state     Link type     Description

            Point-to-Point      1             Point-to-point link
            DS Other*           2             Multi-access link
            Backup*             2             Multi-access link
            DS**                2             Multi-access link

              *If a full adjacency has been formed with the designated
               switch.

             **If a full adjacency has been formed with at least one
               other switch on the link.

               Table 4: Link Types in a Switch Link Advertisement

   Each link in the advertisement is also assigned a link identifier
   based on its link type.  In general, this value identifies another
   switch that also originates advertisements for the link, thereby
   providing a key for accessing other link state advertisements for the
   link.  The relationship between link type and ID is shown in Table 5.


             Type  Description           Link ID

            1     Point-to-point link   Switch ID of neighbor switch
            2     Multi-access link     Switch ID of designated switch

               Table 5: Link IDs in a Switch Link Advertisement


   In addition to a type and an identifier, the description of each link
   specifies the interface ID of the associated network link.

   Finally, each link description includes the cost of sending a packet
   over the link.  This output cost is expressed in the link state
   metric and must be greater than zero.

   To illustrate the format of a switch link advertisement, consider the
   switch fabric shown in Figure 4.

   In this example, switch SW1 has 5 neighboring switches (shown as
   boxes) distributed over 3 network links (shown as lines).  The base
   MAC address of each switch is also shown adjacent to each box.  On
   switch SW1, ports 01 and 02 attach to point-to-point network links,




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   while port 03 attaches to a multi-access network link with three
   attached switches.  The interface state of each port is shown next to
   the line representing the corresponding link.


                            00-00-1d-22-23-c5
                                +-------+
                                |  SW2  |
                                +-------+
                                    |
                                    | Point-to-Point
                                    |
                                    | 01
       +-------+    Loopback    +-------+
       |  SW3  |----------------|  SW1  | 00-00-1d-1f-05-81
       +-------+             02 +-------+
   00-00-1d-17-35-a4                | 03
                                    |
                                    | DS Other
                                    |
               +--------------------+--------------------+
               |                    |                    |
               | DS Other           | Backup             | DS
               |                    |                    |
           +-------+            +-------+            +-------+
           |  SW4  |            |  SW5  |            |  SW6  |
           +-------+            +-------+            +-------+
        00-00-1d-4a-26-b3    00-00-1d-4a-27-1c    00-00-1d-7e-84-2e


                    Figure 4: Sample Switch Fabric

   The switch link advertisement generated by switch SW1 would contain
   the following data items:

      ; switch link advertisement for switch SW1

      LS age = 0               ; always true on origination
      Options = (T-bit|E-bit)  ; options
      LS type = 1              ; this is a switch link advert











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                               ; SW1's switch ID
      Link State ID = 00-00-1d-1f-05-81-00-00-00-00
      Advertising switch = 00-00-1d-1f-05-81-00-00-00-00
      # links = 2

         ; link on interface port 1
         Link ID = 00-00-1d-22-23-c5-00-00-00-00    ; switch ID

         Link Data = 00-00-1d-1f-05-81-00-00-00-01  ; interface ID
         Type = 1                                   ; pt-to-pt link
         # other metrics = 0                        ; TOS 0 only
         TOS 0 metric = 1

         ; link on interface port 2 is not fully functional

         ; link on interface port 3
         Link ID = 00-00-1d-7e-84-2e-00-00-00-00    ; switch ID of DS
         Link Data = 00-00-1d-1f-05-81-00-00-00-03  ; interface ID
         Type = 2                                   ; multi-access
         # other metrics = 0                        ; TOS 0 only
         TOS 0 metric = 2

   (See Section 11.2 for a detailed description of the format of a
   switch link advertisement.)

8.1.2 Network Link Advertisements

   Network link advertisements are used to describe the switches
   attached to each multi-access network link.

   Note:  Network link advertisements are not generated for point-to-
   point links.

   A network link advertisement is originated by the designated switch
   for the associated multi-access link once the switch has established
   a full adjacency with at least one other switch on the link.  Each
   advertisement lists the switch IDs of those switches that are fully
   adjacent to the designated switch.  The designated switch includes
   itself in this list.

   To illustrate the format of a network link advertisement, consider
   again the switch fabric shown in Figure 4.  In this example, network
   link advertisements will be generated only by switch SW6, the
   designated switch of the multi-access network link between switches
   SW1 and switches SW4, SW5, and SW6.

   The network link advertisement generated by switch SW6 would contain
   the following data items:



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      ; network link advertisement for switch SW6

      LS age = 0               ; always true on origination
      Options = (T-bit|E-bit)  ; options
      LS type = 2              ; this is a network link advert

                                 ; SW6's switch ID
      Link State ID = 00-00-1d-73-84-2e-00-00-00-00
      Advertising switch = 00-00-1d-73-84-2e-00-00-00-00

         Attached switch = 00-00-1d-7e-84-2e-00-00-00-00
         Attached switch = 00-00-1d-4a-26-b3-00-00-00-00
         Attached switch = 00-00-1d-1f-05-81-00-00-00-00
         Attached switch = 00-00-1d-4a-27-1c-00-00-00-00

      (See Section 11.3 for a detailed description of the format of a
      network link advertisement.)

8.2 Distributing Link State Advertisements

   Link state advertisements are distributed throughout the switch
   fabric encapsulated within Link State Update packets.  A single Link
   State Update packet may contain several distinct advertisements.

   To make the distribution process reliable, each advertisement must be
   explicitly acknowledged in a Link State Acknowledgment packet.  Note,
   however, that multiple acknowledgments can be grouped together into a
   single Link State Acknowledgment packet. A sending switch retransmits
   unacknowledged Link State Update packets at regular intervals until
   they are acknowledged.

   The remainder of this section is structured as follows:

   o  Section 8.2.1 presents an overview of the distribution process.

   o  Section 8.2.2 describes how an incoming Link State Update packet
      is processed.

   o  Section 8.2.3 describes how a Link State Packet is forwarded --
      both by the originating switch and an intermediate receiving
      switch.

   o  Section 8.2.4 describes how advertisements are installed into the
      local database.

   o  Section 8.2.5 describes the retransmission of unacknowledged
      advertisements.




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    o  Section 8.2.6 describes how advertisements are acknowledged.

8.2.1 Overview

   The philosophy behind the distribution of link state advertisements
   is based on the concept of adjacencies -- that is, each switch is
   only required to remain synchronized with its adjacent neighbors.

   When a switch originates a new instance of a link state
   advertisement, it formats the advertisement into a Link State Update
   packet and floods the packet out each fully operational interface --
   that is, each interface with a state greater than Waiting.  However,
   only those neighbors that are adjacent to the sending switch need to
   process the packet.

   The sending switch indicates which of its neighbor switches should
   process the advertisement by specifying a particular multicast
   destination in the network layer address information (see Section
   10.3).  The sending switch sets the value of the network layer
   destination switch ID field according to the state of the interface
   over which the packet is sent:

   o  If the interface state is Point-to-Point, DS, or Backup, the
      switch is adjacent to all other switches on the link and all
      neighboring switches must process the packet.  Therefore, the
      destination field is set to the multicast switch ID
      AllSPFSwitches.

   o  If the interface state is DS Other, the switch is only adjacent to
      the designated switch and the backup designated switch and only
      those two neighboring switches must process the packet.
      Therefore, the destination field is set to the multicast switch ID
      AllDSwitches.

   A similar logic is used when a switch receives a Link State Update
   packet containing a new instance of a link state advertisement.
   After processing and acknowledging the packet, the receiving switch
   forwards the Link State Update packet as

   o  On the interface over which the original Link State Update packet
      was received:










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      o  If the receiving switch is the designated switch for the
         attached network link, the packet is forwarded to all other
         switches on the link.  (The destination field is set to
         AllSPFSwitches.)  The originating switch will recognize that it
         was the advertisement originator and discard the packet.

      o  If the receiving switch is not the designated switch for the
         attached network link, the packet is not sent back out the
         interface over which it was received.

   o  On all other interfaces:

      o  If the receiving switch is the designated switch for the
         attached network link, the packet is forwarded to all switches
         on the link.  (The destination field is set to AllSPFSwitches.)

      o  If the receiving switch is neither the designated switch or the
         backup designated switch for the attached network link, the
         packet is forwarded only to the designated switch and the
         backup designated switch.  (The destination field is set to
         AllDSwitches.)

   Each Link State Update packet is forwarded and processed in this
   fashion until all switches in the fabric have received notification
   of the new instance of the link state advertisement.

8.2.2 Processing an Incoming Link State Update Packet

   When the a Link State Update packet is received, it is first
   subjected to a number of consistency checks.  In particular, the Link
   State Update packet is associated with a specific neighbor. If the
   state of that neighbor is less than Exchange, the entire Link State
   Update packet is discarded.

   Each link state advertisement contained in the packet is processed as
   follows:

   1. Validate the advertisement's link state checksum and type. If the
      checksum is invalid or the type is unknown, discard the
      advertisement without acknowledging it.

   2. If the advertisement's age is equal to MaxAge and there is
      currently no instance of the advertisement in the local link state
      database, then do the following:







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      a) Acknowledge the advertisement by sending a Link State
         Acknowledgment packet to the sending neighbor (see Section
         8.2.6).

      b) Purge all outstanding requests for equal or previous instances
         of the advertisement from the sending neighbor's Link State
         Request list.

      c) If the neighbor is Exchange or Loading, install the
         advertisement in the link state database (see Section 8.2.4).
         Otherwise, discard the advertisement.

   3. If the advertisement's age is equal to MaxAge and there is an
      instance of the advertisement in the local link state database,
      then do the following:

      a) If the advertisement is listed in the link state retransmission
         list of any neighbor, remove the advertisement from the
         retransmission list(s) and delete the database copy of the
         advertisement.

      b) Discard the received (MaxAge) advertisement without
         acknowledging it.

   4. If the advertisement's age is less than MaxAge, attempt to locate
      an instance of the advertisement in the local link state database.
      If there is no database copy of this advertisement, or the
      received advertisement is more recent than the database copy (see
      Section 7.1.1), do the following:

      a) If there is already a database copy, and if the database copy
         was installed less than MinLSInterval seconds ago, discard the
         new advertisement without acknowledging it.

      b) Otherwise, forward the new advertisement out some subset of the
         local interfaces (see Section 8.2.3).  Note whether the
         advertisement was sent back out the receiving interface for
         later use by the acknowledgment process.

      c) Remove the current database copy from the Link state
         retransmission lists of all neighbors.

      d) Install the new advertisement in the link state database,
         replacing the current database copy.  (Note that this may cause
         the calculation of the set of best paths to be scheduled.  See
         Section 9.)  Timestamp the new advertisement with the time that
         it was received to prevent installation of another instance
         within MinLSInterval seconds.



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      e) Acknowledge the advertisement, if necessary, by sending a Link
         State Acknowledgment packet back out the receiving interface.
         (See Section 8.2.6.)

      f) If the link state advertisement was initially advertised by the
         local switch itself, advance the advertisement sequence number
         and issue a new instance of the advertisement. (Receipt of a
         newer instance of an advertisement means that the local copy of
         the advertisement is left over from before the last time the
         switch was restarted.)

   5. If the received advertisement is the same instance as the database
      copy (as determined by the algorithm described in Section 7.1.1),
      do the following:

      a) If the advertisement is listed in the neighbor's link state
         retransmission list, the local switch is expecting an
         acknowledgment for this advertisement.  Treat the received
         advertisement as an implied acknowledgment, and remove the
         advertisement from the link state retransmission list. Note
         this implied acknowledgment for later use by the acknowledgment
         process (Section 8.2.6).

      b) Acknowledge the advertisement, if necessary, by sending a Link
         State Acknowledgment packet back out the receiving interface.
         (See Section 8.2.6.)

   If the database copy of the advertisement is more recent than the
      instance just received, do the following:

      a) Determine whether the instance is listed in the neighbor link
         state request list.  If so, an error has occurred in the
         database exchange process.  Restart the database exchange
         process by generating a neighbor BadLSReq event for the sending
         neighbor and terminate processing of the Link State Update
         packet.

      b) Otherwise, generate an unusual event to network management and
         discard the advertisement.

8.2.3 Forwarding Link State Advertisements

   When a new instance of an advertisement is originated or after an
   incoming advertisement has been processed, the switch must decide
   over which interfaces and to which neighbors the advertisement will
   be forwarded.  In some instances, the switch may decide not to
   forward the advertisement over a particular interface because it is
   able to determine that the neighbors on that attached link have or



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   will receive the advertisement from another switch on the link.

   The decision of whether to forward an advertisement over each of the
   switch's interfaces is made as follows:

   1. Each neighboring switch attached to the interface is examined to
      determine whether it should receive and process the new
      advertisement.  For each neighbor, the following steps are
      executed:

      a) If the neighbor state is less than Exchange, the neighbor need
         not receive or process the new advertisement.

      b) If the neighbor state is Exchange or Loading, examine the link
         state request list associated with the neighbor.  If an
         instance of the new advertisement is on the list, the
         neighboring switch already has an instance of the
         advertisement.  Compare the new advertisement to the neighbor's
         copy:

         o  If the new advertisement is less recent, the neighbor need
            not receive or process the new advertisement.

         o  If the two copies are the same instance, delete the
            advertisement from the link state request list.  The
            neighbor need not receive or process the new advertisement
            [7].

         o  Otherwise, the new advertisement is more recent.  Delete the
            advertisement from the link state request list.  The
            neighbor may need to receive and process the new
            advertisement.

      c) If the new advertisement was received from this neighbor, the
         neighbor need not receive or process the advertisement.

      d) Add the new advertisement to the link state retransmission list
         for the neighbor.

   2. The switch must now decide whether to forward the new
      advertisement out the interface.

      a) If the link state advertisement was not added to any of the
         link state retransmission lists for neighbors attached to the
         interface, there is no need to forward the advertisement out
         the interface.





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      b) If the new advertisement was received on this interface, and it
         was received from either the designated switch or the backup
         designated switch, there is no need to forward the
         advertisement out the interface.  Chances are all neighbors on
         the attached network link have also received the advertisement
         already.

      c) If the new advertisement was received on this interface and the
         state of the interface is Point-to-Point, there is no need to
         forward the advertisement since the received advertisement was
         originated by the neighbor switch.

      d) If the new advertisement was received on this interface, and
         the interface state is Backup -- that is, the switch itself is
         the backup designated switch -- there is no need to forward the
         advertisement out the interface.  The designated switch will
         distribute advertisements on the attached network link.

      e) Otherwise, the advertisement must be forwarded out the
         interface.

      To forward a link state advertisement, the switch first increments
      the advertisement's age by InfTransDelay seconds to account for
      the transmission time over the link.  The switch then copies the
      advertisement into a Link State Update packet

      Forwarded advertisements are sent to all adjacent switches
      associated with the interface.  If the interface state is Point-
      to-Point, DS, or Backup, the destination switch ID field of the
      network layer address information is set to the multicast switch
      ID AllSPFSwitches.  If the interface state is DS Other, the
      destination switch ID field is set to the multicast switch ID
      AllDSwitches.

8.2.4 Installing Link State Advertisements in the Database

   When a new link state advertisement is installed into the link state
   database, as the result of either originating or receiving a new
   instance of an advertisement, the switch must determine whether the
   best paths need to be recalculated.  To make this determination, do
   the following:

   1. Compare the contents of the new instance with the contents of the
      old instance (assuming the older instance is available). Note that
      this comparison does not include any data from the link state
      header.  Differences in fields within the header (such as the
      sequence number and checksum, which are guaranteed to be different
      in different instances of an advertisement) are of no consequence



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      when deciding whether or not to recalculate the set of best paths.

   2. If there are no differences in the contents of the two
      advertisement instances, there is no need to recalculate the set
      of best paths.

   3. Otherwise, the set of best paths must be recalculated.

   Note also that the older instance of the advertisement must be
   removed from the link state database when the new advertisement is
   installed.  The older instance must also be removed from the link
   state retransmission lists of all neighbors.

8.2.5 Retransmitting Link State Advertisements

   When a switch sends a link state advertisement to an adjacent
   neighbor, it records the advertisement in the neighbor's link state
   retransmission list.  To ensure the reliability of the distribution
   process, the switch continues to periodically retransmit the
   advertisements specified in the list until they are acknowledged.

   The interval timer used to trigger retransmission of the
   advertisements is set to RxmtInterval seconds, as found in the
   interface data structure. Note that if this value is too low,
   needless retransmissions will ensue.  If the value is too high, the
   speed with which the databases synchronize across adjacencies may be
   affected if there are lost packets.

   When the interval timer expires, entries in the retransmission list
   are formatted into one or more Link State Update packets. (Remember
   that multiple advertisements can fit into a single Link State Update
   packet.)  The age field of each advertisement is incremented by
   InfTransDelay, as found in the interface data structure, before the
   advertisement is copied into the outgoing packet.

   Link State Update packets containing retransmitted advertisements are
   always sent directly to the adjacent switch. That is, the destination
   field of the network layer addressing information is set to the
   switch ID of the neighboring switch.

   If the adjacent switch goes down, retransmissions will continue until
   the switch failure is detected and the adjacency is torn down by the
   VLSP discovery process.  When the adjacency is torn down, the link
   state retransmission list is cleared.







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8.2.6 Acknowledging Link State Advertisements

   Each link state advertisement received by a switch must be
   acknowledged.  In most cases, this is done by sending a Link State
   Acknowledgment packet.  However, acknowledgments can also be done
   implicitly by sending Link State Update packets (see step 4a of
   Section 8.2.2).

   Multiple acknowledgments can be grouped together into a single Link
   State Acknowledgment packet.

   Sending an acknowledgment

      Link State Acknowledgment packets are sent back out the interface
      over which the advertisement was received.  The packet can be sent
      immediately to the sending neighbor, or it can be delayed and sent
      when an interval timer expires.

      o  Sending delayed acknowledgments facilitates the formatting of
         multiple acknowledgments into a single packet.  This enables a
         single packet to send acknowledgments to several neighbors at
         once by using a multicast switch ID in the destination field of
         the network layer addressing information (see below).  Delaying
         acknowledgments also randomizes the acknowledgment packets sent
         by the multiple switches attached to a multi-access network
         link.

         Note that the interval used to time delayed acknowledgments
         must be short (less than RxmtInterval) or needless
         retransmissions will ensue.

         Delayed acknowledgments are sent to all adjacent switches
         associated with the interface.  If the interface state is
         Point-to-Point, DS, or Backup, the destination field of the
         network layer addressing information is set to the multicast
         switch ID AllSPFSwitches.  If the interface state is DS Other,
         the destination field is set to the multicast switch ID
         AllDSwitches.

      o  Immediate acknowledgments are sent directly to a specific
         neighbor in response to the receipt of duplicate link state
         advertisements.  These acknowledgments are sent immediately
         when the duplicate is received.

      The method used to send a Link State Acknowledgment packet --
      either delayed or immediate -- depends on the circumstances
      surrounding the receipt of the advertisement, as shown in Table 6.
      Note that switches with an interface state of Backup send



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      acknowledgments differently than other switches because they play
      a slightly different role in the distribution process (see Section
      8.2.3).

                                  Action taken in state
      Circumstances           Backup               Other states

      Advertisement was       No ack sent          No ack sent
      forwarded back out
      receiving interface

      Advertisement is        Delayed ack sent     Delayed ack
      more recent than        if advertisement     sent
      database copy, but      received from DS,
      was not forwarded       else do nothing
      back out receiving
      interface

      Advertisement was a     Delayed ack sent     No ack sent
      duplicate treated       if advertisement
      as an implied acknow-   received from DS,
      ledgment (step 4a of    else do nothing
      Section 8.2.2)

      Advertisement was a     Immediate ack        Immediate ack
      duplicate not treated   sent                 sent
      as an implied acknow-
      ledgment

      Advertisement age       Immediate ack        Immediate ack
      equal to MaxAge and     sent                 sent
      no current instance
      found in database

               Table 6: Sending Link State Acknowledgments

   Receiving an acknowledgment

      When the a Link State Acknowledgment packet is received, it is
      first subjected to a number of consistency checks.  In particular,
      the packet is associated with a specific neighbor. If the state of
      that neighbor is less than Exchange, the entire Link State
      Acknowledgment packet is discarded.

      Each acknowledgment contained in the packet is processed as
      follows:





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      o  If the advertisement being acknowledged has an instance in the
         link state retransmission list for the sending neighbor, do the
         following:

         o  If the acknowledgment is for the same instance as that
            specified in the list (as determined by the procedure
            described in Section 7.1.1), remove the instance from the
            retransmission list.

         o  Otherwise, log the acknowledgment as questionable.

8.3 Aging the Link State Database

   Each link state advertisement has an age field, containing the
   advertisement's age, expressed in seconds.  When the advertisement is
   copied into a Link State Update packet for forwarding out a
   particular interface, the age is incremented by InfTransDelay seconds
   to account for the transmission time over the link.  An
   advertisement's age is never incremented past the value MaxAge.
   Advertisements with an age of MaxAge are not used to calculate best
   paths.

   If a link state advertisement's age reaches MaxAge, the switch
   flushes the advertisement from the switch fabric by doing the
   following:

   o  Originate a new instance of the advertisement with the age field
      set to MaxAge.  The distribution process will eventually result in
      the advertisement being removed from the retransmission lists of
      all switches in the fabric.

   o  Once the advertisement is no longer contained in the link state
      retransmission list of any neighbor and no neighbor is in a state
      of Exchange or Loading, remove  the advertisement from the local
      link state database.

8.3.1 Premature Aging of Advertisements

   A link state advertisement can be prematurely flushed from the switch
   fabric by forcing its age to MaxAge and redistributing the
   advertisement.

   A switch that was previously the designated switch for a multi-access
   network link but has lost that status due to a failover to the backup
   designated switch prematurely ages the network link advertisements it
   originated for the link.





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   Premature aging also occurs when an advertisement's sequence number
   must wrap -- that is, when the current advertisement instance has a
   sequence number of 0x7fffffff.  In this circumstance, the
   advertisement is prematurely aged so that the next instance of the
   advertisement can be originated with a sequence number of 0x80000001
   and be recognized as the most recent instance.

   A switch may only prematurely age those link state advertisements for
   which it is the advertising switch.

9. Calculating the Best Paths

   Once an adjacency has been formed and the two switches have
   synchronized their databases, each switch in the adjacency calculates
   the best path(s) to all other switches in the fabric, using itself as
   the root of each path.  In this context, "best" path means that path
   with the lowest total cost metric across all hops.  If there are
   multiple paths with the same (lowest) total cost metric, they are all
   calculated.  Best paths are stored in the area data structure.

   Paths are calculated using the well-known Dijkstra algorithm. For a
   detailed description of this algorithm, the reader is referred to
   [Perlman], or any of a number of standard textbooks dealing with
   network routing.

   Note that whenever there is a change in an adjacency relationship, or
   any change that alters the topology of the switch fabric, the set of
   best paths must be recalculated.

10. Protocol Packets

   This section describes VLS protocol packets and link state
   advertisements.


















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   There are five distinct VLSP packet types, as listed in Table 7.

     Type  Packet Name       Function              Description

     1     Hello             Select DS/Backup DS   Section 10.6.1
     2     Database          Summarize database
             Description     contents              Section 10.6.2
     3     LS Request        Database download     Section 10.6.3
     4     LS Update         Database update       Section 10.6.4
     5     LS Ack            Flooding acknow-
                             ledgment              Section 10.6.5

                      Table 7: VLSP Packet Types


   All VLSP packets are encapsulated within a standard ISMP packet, with
   the VLS packet carried in the ISMP message body.  The ISMP packet is
   described in Section 10.1.

   Since it is important that the link state databases remain
   synchronized throughout the switch fabric, processing of both
   incoming and outgoing routing protocol packets should take priority
   over ordinary data packets.  Section 10.2 describes packet
   processing.

   All VLSP packets begin with network layer addressing information,
   described in Section 10.3, followed by a standard header, described
   in Section 10.4.

   With the exception of Hello packets, all VLSP packets deal with lists
   of link state advertisements.  The format of a link state
   advertisement is described in Section 11.

10.1 ISMP Packet Format

   All VLSP packets are encapsulated within a standard ISMP packet. ISMP
   packets are of variable length and have the following general
   structure:

   o  Frame header
   o  ISMP packet header
   o  ISMP message body









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10.1.1 Frame Header

   ISMP packets are encapsulated within an IEEE 802-compliant frame
   using a standard header as shown below:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      +      Destination address      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   04 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        Source address         +
   08 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   12 |             Type              |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   16 |                                                               |
      +                                                               +
      :                                                               :

   Destination address

      This 6-octet field contains the Media Access Control (MAC) address
      of the multicast channel over which all switches in the fabric
      receive ISMP packets.  The destination address of all ISMP packets
      contain a value of 01-00-1D-00-00-00.

   Source address

      This 6-octet field contains the physical (MAC) address of the
      switch originating the ISMP packet.

   Type

      This 2-octet field identifies the type of data carried within the
      frame.  The type field of ISMP packets contains the value 0x81FD.















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10.1.2 ISMP Packet Header

   The ISMP packet header consists of 6 octets, as shown below:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |///////////////////////////////////////////////////////////////|
      ://////// Frame header /////////////////////////////////////////:
      +//////// (14 octets)  /////////+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   12 |///////////////////////////////|            Version            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   16 |       ISMP message type       |        Sequence number        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   20 |                                                               |
      +                                                               +
      :                                                               :


   Frame header

      This 14-octet field contains the frame header.

      Version

      This 2-octet field contains the version number of the InterSwitch
      Message Protocol to which this ISMP packet adheres.  This document
      describes ISMP Version 2.0.           ISMP message type

      This 2-octet field contains a value indicating which type of ISMP
      message is contained within the message body.  Valid values are as
      follows:

         1    (reserved)
         2    Interswitch Keepalive messages
         3    Interswitch Link State messages
         4    Interswitch Spanning Tree BPDU messages and
              Interswitch Remote Blocking messages
         5    Interswitch Resolve and New User messages
         6    (reserved)
         7    Tag-Based Flood messages
         8    Interswitch Tap messages

      All VLS protocol messages have an ISMP message type of 3.







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   Sequence number

      This 2-octet field contains an internally generated sequence
      number used by the various protocol handlers for internal
      synchronization of messages.

10.1.3 ISMP Message Body

   The ISMP message body is a variable-length field containing the
   actual data of the ISMP message.  The length and content of this
   field are determined by the value found in the message type field.
   VLSP packets are contained in the ISMP message body.

10.2 VLSP Packet Processing

   Note that with the exception of Hello packets, VLSP packets are sent
   only between adjacent neighbors.  Therefore, all packets travel a
   single hop.

   VLSP does not support fragmentation and reassembly of packets.
   Therefore, packets containing lists of link state advertisements or
   advertisement headers must be formatted such that they contain only
   as many advertisements or headers as will fit within the size
   constraints of a standard ethernet frame.

   When a protocol packet is received by a switch, it must first pass
   the following criteria before being accepted for further processing:

   o  The checksum number must be correct.

   o  The destination switch ID (as found in the network layer address
      information) must be the switch ID of the receiving switch, or one
      of the multicast switch IDs AllSPFSwitches or AllDSwitches.

      If the destination switch ID is the multicast switch ID
      AllDSwitches, the state of the receiving interface must be Point-
      to-Point, DS, or Backup.

   o  The source switch ID (as found in the network layer address
      information) must not be that of the receiving switch.  (That is,
      locally originated packets should be discarded.)

   At this point, if the packet is a Hello packet, it is accepted for
   further processing.







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   Since all other packet types are only sent between adjacent
   neighbors, the packet must have been sent by one of the switch's
   active neighbors.  If the source switch ID matches the switch ID of
   one of the receiving switch's active neighbors (as stored in the
   interface data structure associated with the inport interface), the
   packet is accepted for further processing.  Otherwise, the packet is
   discarded.

10.3 Network Layer Address Information

   As mentioned in Section 2.2.1, portions of the VLS protocol (as
   derived from OSPF) are dependent on certain network layer addresses
   -- in particular, the AllSPFSwitches and AllDSwitches multicast
   addresses that drive the distribution of link state advertisements
   throughout the switch fabric.  In order to facilitate the
   implementation of the protocol at the physical MAC layer, network
   layer address information is encapsulated in the VSLP packets.  This
   information immediately follows the ISMP frame and packet header and
   immediately precedes the VLSP packet header, as shown below:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      :                  frame header / ISMP header                   :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :                      Unused (20 octets)                       :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   20 |                                                               |
      +                       Source switch ID                        +
   24 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   28 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   32 |                                                               |
      +                     Destination switch ID                     +
   36 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   40 |                                                               |
      :                          VLSP header                          :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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   Source switch ID

      This 10-octet field contains the switch ID of the sending switch.

   Destination switch ID

      This 10-octet field contains the switch ID of the packet
      destination.  The value here is set as follows:

      o  Hello packets are addressed to the multicast switch ID
         AllSPFSwitches.

      o  The designated switch and the backup designated switch address
         initial Link State Update packets and Link State Acknowledgment
         packets to the multicast switch ID AllSPFSwitches.

      o  All other switches address initial Link State Update packets
         and Link State Acknowledgment packets to the multicast switch
         ID AllDSwitches.

      o  Retransmissions of Link State Update packets are always
         addressed directly to the nonresponding switch.

      o  Database Description packets and Link State Request are always
         addressed directly to the other switch participating in the
         database exchange process.

   VLSP header

      This 30-octet field contains the VLSP standard header.  See
      Section 10.4.

10.4 VLSP Packet Header

   Every VLSP packet starts with a common 30-octet header.  This header,
   along with the data found in the network layer address information,
   contains all the data necessary to determine whether the packet
   should be accepted for further processing. (See Section 10.1.)

   The format of the VLSP header is shown below.  Note that the header
   starts at offset 36 of the ISMP message body, following the network
   layer address information.









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        0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      :                  frame header / ISMP header                   :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :               Network layer address information               :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   40 |    (unused)   |     Type      |         Packet length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   44 |                                                               |
      +                       Source switch ID                        +
   48 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   52 |                               |         Area ID . . .         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   56 |         Area ID . . .         |           Checksum            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   60 |            Autype             |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        Authentication         +
   64 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   68 |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      This 1-octet field contains the packet type.  Possible values are
      as follows:

         1   Hello
         2   Database Description
         3   Link State Request
         4   Link State Update
         5   Link State Acknowledgment

   Packet length

      This 2-octet field contains the length of the protocol packet, in
      bytes, calculated from the start of the VLSP header, at offset 20
      of the ISMP message body.  If the packet length is not an integral
      number of 16-bit words, the packet is padded with an octet of zero
      (see the description of the checksum field, below).




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   Switch ID

      This 10-octet field contains the switch ID of the sending switch.

   Area ID

      This 4-octet field contains the area identifier.  Since VLSP does
      not support multiple areas, the value here is always zero.

   Checksum

      This 2-octet field contains the packet checksum value.  The
      checksum is calculated as the 16-bit one's complement of the one's
      complement sum of all the 16-bit words in the packet, beginning
      with the VLSP header, excluding the authentication field.  If the
      packet length is not an integral number of 16-bit words, the
      packet is padded with an octet of zero before calculating the
      checksum.

   AuType

      This 2-octet field identifies the authentication scheme to be used
      for the packet.  Since authentication is not supported by this
      version of VLSP, this field contains zero.

   Authentication

      This 8-octet field is reserved for use by the authentication
      scheme.  Since authentication is not supported by this version of
      VLSP, this field contains zeroes.

10.5 Options Field

   Hello packets and Database Description packets, as well as link state
   advertisements, contain a 1-octet options field.  Using this field, a
   switch can communicate its optional capabilities to other VLSP
   switches.  The receiving switch can then choose whether or not to
   support those optional capabilities.  Thus, switches of differing
   capabilities potentially can be mixed within a single VLSP routing
   domain.

   Two optional capabilities are currently defined in the options field:
   routing based on Type of Service (TOS) and support for external
   routing beyond the local switch fabric.  These two capabilities are
   specified in the options field as shown below.






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                             +-+-+-+-+-+-+-+-+
                             |0|0|0|0|0|0|E|T|
                             +-+-+-+-+-+-+-+-+

                             The options field

   T-bit

      The T-bit specifies the switch's Type of Service (TOS) capability.
      If the T-bit is set, the switch supports routing based on nonzero
      types of service.

   E-bit

      The E-bit specifies the switch's external routing capability. If
      the E-bit is set, the switch supports external routing.

   Note:  The current version of VLSP supports neither of these
   capabilities.  Therefore, both the T-bit and the E-bit are clear and
   the options field contains a value of zero.

10.6 Packet Formats

   This section contains detailed descriptions of the five VLS protocol
   packets.

10.6.1 Hello Packets

   Hello packets are sent periodically over multi-access switch
   interfaces in order to discover and maintain neighbor relationships.

   Note:  Hello packets are not sent over point-to-point network links.
   For point-to-point links, the VLS protocol relies on the VlanHello
   protocol [IDhello] to notify it of neighboring switches.

   Since all switches connected to a common network link must agree on
   certain interface parameters, these parameters are included in each
   Hello packet.  A switch receiving a Hello packet that contains
   parameters inconsistent with its own view of the interface will not
   establish a neighbor relationship with the sending switch.

   The format of a Hello packet is shown below.









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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    00 |                                                               |
       :              Network layer addressing / VLSP header           :
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    70 |                      (unused -- must be 0)                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    74 |         HelloInt              |    Options    |   Priority    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    78 |                            DeadInt                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    82 |                                                               |
       +                      Designated switch ID                     +
    86 |                                                               |
       +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    90 |                               |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    94 |                                                               |
       +                   Backup designated switch ID                 +
    98 |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   102 |                                                               |
       +                                                               +
       :                          Neighbor list                        :
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 1.

   HelloInt

      This 2-octet field contains the interval, in seconds, at which
      this switch sends Hello packets.

   Options

      This 1-octet field contains the optional capabilities supported by
      the switch, as described in Section 10.5.






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   Priority

      This 1-octet field contains the switch priority used in selecting
      the designated switch and backup designated switch (see Section
      6.3.1).  If the value here is zero, the switch is ineligible to
      become the designated switch or the backup designated switch.

   DeadInt

      This 4-octet field contains the length of time, in seconds, that
      neighboring switches will wait before declaring the interface down
      once they stop receiving Hello packets over the interface.  The
      value here is equal to the value of SwitchDeadInterval, as found
      in the interface data structure.

   Designated switch

      This 10-octet field contains the switch ID of the designated
      switch for this network link, as currently understood by the
      sending switch.  This value is set to zero if the designated
      switch selection process has not yet begun.

   Backup designated switch

      This 10-octet field contains the switch ID of the backup
      designated switch for the network link, as currently understood by
      the sending switch.  This value is set to zero if the backup
      designated switch selection process has not yet begun.

   Neighbor list

      This variable-length field contains a list of switch IDs of each
      switch from which the sending switch has received a valid Hello
      packet within the last SwitchDeadInterval seconds.

10.6.2 Database Description Packets

   Database Description packets are exchanged while an adjacency is
   being formed between two neighboring switches and are used to
   describe the contents of the topological database.  For a complete
   description of the database exchange process, see Section 7.2.

   The format of a Database Description packet is shown below.








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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :              Network layer addressing / VLSP header           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   70 |     (unused -- must be 0)     |    Options    |     Flags     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   74 |                        Sequence number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   78 |                                                               |
      +                                                               +
      :                 Link state advertisement headers              :
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 2.

   Options

      This 1-octet field contains the optional capabilities supported by
      the switch, as described in Section 10.5.

   Flags

      This 1-octet field contains a set of bit flags that are used to
      coordinate the database exchange process.  The format of this
      octet is as follows:

                          +-+-+-+-+-+-+-+-+
                          |0|0|0|0|0|I|M|MS
                          +-+-+-+-+-+-+-+-+

   I-bit (Init)

      The I-bit is used to signal the start of the exchange.  It is set
      while the two switches negotiate the master/slave relationship and
      the starting sequence number.






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   M-bit (More)

      The M-bit is set to indicate that more Database Description
      packets to follow.

   MS-bit (Master/Slave)

      The MS-bit is used to indicate which switch is the master of the
      exchange.  If the bit is set, the sending switch is the master
      during the database exchange process.  If the bit is clear, the
      switch is the slave.

   Sequence number

      This 4-octet field is used to sequence the Database Description
      packets during the database exchange process.  The two switches
      involved in the exchange process agree on the initial value of the
      sequence number during the master/slave negotiation.  The number
      is then incremented for each Database Description packet in the
      exchange.

      To acknowledge each Database Description packet sent by the
      master, the slave sends a Database Description packet that echoes
      the sequence number of the packet being acknowledged.

   Link state advertisement headers

      This variable-length field contains a list of link state headers
      that describe a portion of the master's topological database.
      Each header uniquely identifies a link state advertisement and its
      current instance.  (See Section 11.1 for a detailed description of
      a link state advertisement header.)  The number of headers
      included in the list is calculated implicitly from the length of
      the packet, as stored in the VLSP packet header (see Section
      10.4).

10.6.3 Link State Request Packets

   Link State Request packets are used to request those pieces of the
   neighbor's database that the sending switch has discovered (during
   the database exchange process) are more up-to-date than instances in
   its own database.  Link State Request packets are sent as the last
   step in bringing up an adjacency.  (See Section 7.3.)

   The format of a Link State Request packet is shown below.






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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :              Network layer addressing / VLSP header           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   70 |                        Link state type                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   74 |                                                               |
      +                         Link state ID                         +
   88 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   82 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   86 |                                                               |
      +                      Advertising switch ID                    +
   90 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   94 |                                                               |
      :                            . . .                              :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 3.

   Link state type

      This 4-octet field contains the link state type of the requested
      link state advertisement, as stored in the advertisement header.

   Link state ID

      This 10-octet field contains the link state ID of the requested
      link state advertisement, as stored in the advertisement header.

   Advertising switch

      This 10-octet field contains the switch ID of advertising switch
      for the requested link state advertisement, as stored in the
      advertisement header.





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      Note that the last three fields uniquely identify the
      advertisement, but not its instance.  The receiving switch will
      respond with its most recent instance of the specified
      advertisement.

      Multiple link state advertisements can be requested in a single
      Link State Request packet by repeating the link state type, ID,
      and advertising switch for each requested advertisement.  The
      number of advertisements requested is calculated implicitly from
      the length of the packet, as stored in the VLSP packet header.

10.6.4 Link State Update Packets

   Link State Update packets are used to respond to a Link State Request
   packet or to advertise a new instance of one or more link state
   advertisements.  Link State Update packets are acknowledged with Link
   State Acknowledgment packets.  For more information on the use of
   Link State Update packets, see Section 7 and Section 8.

   The format of a Link State Update packet is shown below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :              Network layer addressing / VLSP header           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   70 |                        # advertisements                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   74 |                                                               |
      +                                                               +
      :                    Link state advertisements                  :
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 4.

   # advertisements

      This 4-octet field contains the number of link state
      advertisements included in the packet.




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   Link state advertisements

      This variable-length field contains a list of link state
      advertisements.  For a detailed description of the different types
      of link state advertisements, see Section 11.

10.6.5 Link State Acknowledgment Packets

   Link State Acknowledgment Packets are used to explicitly acknowledge
   one or more Link State Update packets, thereby making the
   distribution of link state advertisements reliable.  (See Section
   8.2.6.)

   The format of a Link State Acknowledgment packet is shown below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :              Network layer addressing / VLSP header           :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   70 |                                                               |
      +                                                               +
      :                 Link state advertisement headers              :
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Network layer addressing / VLSP header

      This 70-octet field contains the network layer addressing
      information and the standard VLS protocol packet header.  The
      packet header type field contains a value of 5.

   Link state advertisement headers

      This variable-length field contains a list of link state headers
      that are being acknowledged by this packet.  Each header uniquely
      identifies a link state advertisement and its current instance.
      (See Section 11.1 for a detailed description of a link state
      advertisement header.)  The number of headers included in the list
      is calculated implicitly from the length of the packet, as stored
      in the VLSP packet header (see Section 10.4).







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11. Link State Advertisement Formats

   Link state advertisements are used to describe various pieces of the
   routing topology within the switch fabric.  Each switch in the fabric
   maintains a complete set of all link state advertisements generated
   throughout the fabric.  (Section 8.1 describes the circumstances
   under which a link state advertisement is originated.  Section 8.2
   describes how advertisements are distributed throughout the switch
   fabric.) This collection of advertisements, known as the link state
   (or topological) database, is used to calculate a set of best paths
   to all other switches in the fabric.

   There are two types of link state advertisement, as listed in Table
   8.

        Type   Name            Function             Description

        1      Switch link     Lists all network    Section 11.2
               advertisement   linksattached to
                               a switch

        2      Network link    Lists all adjacen-   Section 11.3
               advertisement   cies on a network
                               link

                Table 8: Link State Advertisement Types

   Each link state advertisement begins with a standard header,
   described in Section 11.1.

11.1 Link State Advertisement Headers

   All link state advertisements begin with a common 32-octet header.
   This header contains information that uniquely identifies the
   advertisement -- its type, link state ID, and the switch ID of its
   advertising switch.  Also, since multiple instances of a link state
   advertisement can exist concurrently in the switch fabric, the header
   contains information that permits a switch to determine which
   instance is the most recent -- the age, sequence number and checksum.

   The format of the link state advertisement header is shown below.










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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |              Age              |    Options    |    LS Type    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   04 |                                                               |
      +                         Link state ID                         +
   08 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   12 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   16 |                                                               |
      +                      Advertising switch ID                    +
   20 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   24 |                         Sequence number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   28 |           Checksum            |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Age

      This 2-octet field contains the time, in seconds, since this
      instance of the link state advertisement was originated.

   Options

      This 1-octet field contains the optional capabilities supported by
      the advertising switch, as described in Section 10.5.

   LS type

      This 1-octet field contains the type of the link state
      advertisement.  Possible values are:

         1   Switch link advertisement
         2   Network link advertisement

   Link state ID

      This 10-octet field identifies the switch that originates
      advertisements for the link.  The content of this field depends on
      the advertisement's type.

      o  For a switch link advertisement, this field contains the switch
         ID of the originating switch




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         o  For a network link advertisement, this field contains the
         switch ID of the designated switch for the link

   Note:  In VLSP, the link state ID of an advertisement is always the
   same as the advertising switch.  This level of redundancy results
   from the fact that OSPF uses additional types of link state
   advertisements for which the originating switch is not the
   advertising switch.

   Advertising switch

      This 10-octet field contains the switch ID of the switch that
      originated the link state advertisement.

   Sequence number

      This 4-octet field is used to sequence the instances of a
      particular link state advertisement.  The number is incremented
      for each new instance.

   Checksum

      This 2-octet field contains the checksum of the complete contents
      of the link state advertisement, excluding the age field.  The
      checksum used is commonly referred to as the Fletcher checksum and
      is documented in [RFC905].  Note that since this checksum is
      calculated for each separate advertisement, a protocol packet
      containing lists of advertisements or advertisement headers will
      contain multiple checksum values.

   Length

      This 2-octet field contains the total length, in octets, of the
      link state advertisement, including the header.

11.2 Switch Link Advertisements

   A switch link advertisement is used to describe all functioning
   network links of a switch, including the cost of using each link.

   Each functioning switch in the fabric originates one, and only one,
   switch link advertisement -- all of the switch's links must be
   described in a single advertisement.  A switch originates its first
   switch link advertisement (containing no links) when it first becomes
   functional.  It then originates a new instance of the advertisement
   each time any of its neighbor states changes such that the contents
   of the advertisement changes.   See Section 8.1 for details on
   originating a switch link advertisement.



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   The format of a switch link advertisement is shown below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :                       Link state header                       :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   32 |      (unused -- must be 0)    |            # links            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   36 |                                                               |
      +                            Link ID                            +
   40 |                                                               |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   44 |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   48 |                                                               |
      +                           Link data                           +
   52 |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   56 |   Link type   |     # TOS     |         TOS 0 metric          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   60 |                                                               |
      :                            . . .                              :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Link state header

      This 32-octet field contains the standard link state advertisement
      header.  The type field contains a 1, and the link state ID field
      contains the switch ID of the advertising switch.

   # links

      This 2-octet field contains the number of links described by this
      advertisement.  This value must be equal to the total number of
      functioning network links attached to the switch.












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   Link ID

      This 10-octet field identifies the other switch that originates
      link state advertisements for the link, providing a key for
      accessing other link state advertisements for the link.  The value
      here is based on the link type, as follows:

      o  For point-to-point links, this field contains the switch ID of
         the neighbor switch connected to the other end of the link.

      o  For multi-access links, this field contains the switch ID of
         the designated switch for the link.

   Link data

      This 10-octet field contains additional data necessary to
      calculate the set of best paths.  Typically, this field contains
      the interface ID of the link.

   Link type

      This 1-octet field contains the type of link being described.
      Possible values are as follows:

         1   Point-to-point link
         2   Multi-access link

   # TOS

      This 1-octet field contains the number of nonzero type of service
      metrics specified for the link.  Since the current version of VLSP
      does not support routing based on nonzero types of service, this
      field contains a value of zero.

   TOS 0 metric

      This 2-octet field contains the cost of using this link for the
      zero TOS.  This value is expressed in the link state metric and
      must be greater than zero.

   Note that the last five fields are repeated for all functioning
   network links attached to the advertising switch.  If the interface
   state of attached link changes, the switch must originate a new
   instance of the switch link advertisement.







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11.3 Network Link Advertisements

   A network link advertisement is originated by the designated switch
   of each multi-access network link.  The advertisement describes all
   switches attached to the link that are currently fully adjacent to
   the designated switch, including the designated switch itself.  See
   Section 8.1 for details on originating a switch link advertisement.

   Network link advertisements are not generated for point-to-point
   network links.

   The format of a network link advertisement is show below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   00 |                                                               |
      :                       Link state header                       :
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   32 |                           (unused)                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   36 |                                                               |
      +                                                               +
      :                          Switch list                          :
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Link state header

      This 32-octet field contains the standard link state advertisement
      header.  The type field contains a 2, and the link state ID field
      contains the switch ID of the designated switch.

   Switch list

      The switch IDs of all switches attached to the network link that
      are currently fully adjacent to the designated switch. The
      designated switch includes itself in this list.

12. Protocol Parameters

   This section contains a compendium of the parameters used in the VLS
   protocol.






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12.1 Architectural Constants

   Several VLS protocol parameters have fixed architectural values. The
   name of each architectural constant follows, together with its value
   and a short description of its function.

   AllSPFSwitches

      The multicast switch ID to which Hello packets and certain other
      protocol packets are addressed, as specified in the destination
      switch ID field of the network layer address information (see
      Section 10.3).  The value of AllSPFSwitches is E0-00-00-05-00-00-
      00-00.

   AllDSwitches

      The multicast switch ID to which Link State Update packets and
      Link State Acknowledgment packets are addressed, as specified in
      the destination switch ID field of the network layer address
      information (see Section 10.3), when they are destined for the
      designated switch or the backup designated switch of a network
      link.  The value of AllDSwitches is E0-00-00-06-00-00-00-00.

   LSRefreshTime

      The interval at which the set of best paths recalculated if no
      other state changes have forced a recalculation.  The value of
      LSRefreshTime is set to 1800 seconds (30 minutes).

   MinLSInterval

      The minimum time between distinct originations of any particular
      link state advertisement.  The value of MinLSInterval is set to 5
      seconds.

   MaxAge

      The maximum age that a link state advertisement can attain. When
      an advertisement's age reaches MaxAge, it is redistributed
      throughout the switch fabric.  When the originating switch
      receives an acknowledgment for the advertisement, indicating that
      the advertisement has been removed from all neighbor Link state
      retransmission lists, the advertisement is removed from the
      originating switch's database.  Advertisements having age MaxAge
      are not used to calculate the set of best paths.  The value of
      MaxAge must be greater than LSRefreshTime.  The value of MaxAge is
      set to 3600 seconds (1 hour).




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   MaxAgeDiff

      The maximum time disparity in ages that can occur for a single
      link state instance as it is distributed throughout the switch
      fabric.  Most of this time is accounted for by the time the
      advertisement sits on switch output queues (and therefore not
      aging) during the distribution process. The value of MaxAgeDiff is
      set to 900 seconds (15 minutes).

   LSInfinity

      The link state metric value indicating that the destination is
      unreachable.  It is defined to be a binary value of all ones.

12.2 Configurable Parameters

   Many of the switch interface parameters used by VLSP may be made
   configurable if the implementer so desires.  These parameters are
   listed below.  Sample default values are given for some of the
   parameters.

   Note that some of these parameters specify properties of the
   individual interfaces and their attached network links.  These
   parameters must be consistent across all the switches attached to
   that link.

   Interface output cost(s)

      The cost of sending a packet over the interface, expressed in the
      link state metric.  This is advertised as the link cost for this
      interface in the switch's switch link advertisement. The interface
      output cost must always be greater than zero.

   RxmtInterval

      The number of seconds between link state advertisement
      retransmissions for adjacencies established on this interface.
      This value is also used when retransmitting Database Description
      packets and Link State Request packets. This value must be greater
      than the expected round-trip delay between any two switches on the
      attached link.  However, the value should be conservative or
      needless retransmissions will result.  A typical value for a local
      area network would be 5 seconds.








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   InfTransDelay

      The estimated number of seconds it takes to transmit a Link State
      Update packet over this interface.  Link state advertisements
      contained in the Link State Update packet must have their age
      incremented by this amount before transmission.  This value must
      take into account the transmission and propagation delays for the
      interface and must be greater than zero.  A typical value for a
      local area network would be 1 second.

   Switch priority

      An 8-bit unsigned integer.  When two switches attached to the same
      network link contend for selection as the designated switch, the
      switch with the highest priority takes precedence.  If both
      switches have the same priority, the switch with the highest base
      MAC address becomes the designated switch.  A switch whose switch
      priority is set to zero is ineligible to become the designated
      switch on the attached link.

   HelloInterval

      The length of time, in seconds, between the Hello packets that the
      switch sends over the interface.  This value is advertised in the
      switch's Hello packets.  It must be the same for all switches
      attached to a common network link.  The smaller this value is set,
      the faster topological changes will be detected.  However, a
      smaller interval will also generate more routing traffic.  A
      typical value for a local area network would be 10 seconds.

   SwitchDeadInterval

      The length of time, in seconds, that neighboring switches will
      wait before declaring the interface down once they stop receiving
      Hello packets over the interface.  This value is advertised in the
      switch's Hello packets.  It must be the same for all switches
      attached to a common network link and should be some multiple of
      the HelloInterval parameter.  A typical value would be 4 times
      HelloInterval.












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13. End Notes

   [1] During calculation of the set of best paths, a network link
   advertisement must be located based solely on its link state ID.
   Note, however, that the lookup in this case is still well defined,
   since no two network advertisements can have the same link state ID.

   [2] It is instructive to see what happens when the designated switch
   for a network link fails.  Call the designated switch for the link S1
   and the backup designated switch S2.  If switch S1 fails (or its
   interface to the link goes down), the other switches on the link will
   detect S1's absence within SwitchDeadInterval seconds.  All switches
   may not detect this condition at precisely the same time.  The
   switches that detect S1's absence before S2 does will temporarily
   select S2 as both designated switch and backup designated switch.
   When S2 detects that S1 is down, it will move itself to designated
   switch.  At this time, the remaining switch with the highest switch
   priority will be selected as the backup designated switch.

   [3] Note that it is possible for a switch to resynchronize any of its
   fully established adjacencies by setting the neighbor state back to
   ExStart.  This causes the switch on the other end of the adjacency to
   process a SeqNumberMismatch event and also revert to the ExStart
   state.

   [4] When two advertisements have different checksum values, they are
   assumed to be separate instances.  This can occur when a switch
   restarts and loses track of its previous sequence number. In this
   case, since the two advertisements have the same sequence number, it
   is not possible to determine which advertisement is actually newer.
   If the wrong advertisement is accepted as newer, the originating
   switch will originate another instance.

   [5] An instance of an advertisement is originated with an age of
   MaxAge only when it is to be flushed from the database.  This is done
   either when the advertisement has naturally aged to MaxAge, or (more
   typically) when the sequence number must wrap. Therefore, a received
   instance with an age of MaxAge must be processed as the most recent
   in order to flush it properly from the database.

   [6] MaxAgeDiff is an architectural constant that defines the maximum
   disparity in ages, in seconds, that can occur for a single link state
   instance as it is distributed throughout the switch fabric.  If two
   advertisements differ by more than this amount, they are assumed to
   be different instances of the same advertisement. This can occur when
   a switch restarts and loses track of its previous sequence number.





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   [7] This is how the link state request list is emptied, causing the
   neighbor state to change to Full.

14. Security Considerations

   Security concerns are not addressed in this document.

15. References

   [Perlman]    Perlman, R.,  Interconnections: Bridges and Routers.
                Addison-Wesley Publishing Company.  1992.

   [RFC905]     McKenzie, A., "ISO Transport Protocol specification ISO
                DP 8073", RFC 905, April 1984.

   [RFC2328]    Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC1700]    Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
                RFC 1700, October 1994.

   [IDsfvlan]   Ruffen, D., Len, T. and J. Yanacek, "Cabletron's
                SecureFast VLAN Operational Model", RFC 2643, August
                1999.

   [IDhello]    Hamilton, D. and D. Ruffen, "Cabletron's VlanHello
                Protocol Specification", RFC 2641, August 1999.

16. Author's Address

   Laura Kane
   Cabletron Systems, Inc.
   Post Office Box 5005
   Rochester, NH  03866-5005

   Phone:[phone]
   EMail:  [e-mail]















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17.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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