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Chapter 4 - Routing Protocols Used in TCP/IP

Cisco TCP/IP Routing Professional Reference
Chris Lewis
  Copyright © 1999 The McGraw-Hill Companies, Inc.

EIGRP: The Hybrid Protocol
In the early 1990s, Cisco introduced Enhanced IGRP, which uses the same distance vector technology found in IGRP for the underlying metric calculations. What has changed are the route advertising procedures, and the calculation of entries into the routing table. These procedures are like those of a link state protocol. The key components of EIGRP are:
  Neighbor discovery/recovery
  Reliable transport protocol
  DUAL finite state machine
  Variable-length subnet masks
Neighbor discovery is the process by which a router learns of other routers on directly attached links. EIGRP uses small hello packets for neighbor discovery (as long as a router receives hello messages from a neighboring router, it assumes that the neighbor is functioning and they can exchange routing information). The key point is that EIGRP uses partial updates. When the state of a link or router changes, EIGRP sends out only the information necessary to those needing to hear about it, instead of sending the entire routing table to all neighbors. This clearly minimizes the bandwidth used by EIGRP regular update messages when compared to those used by IGRP.
At the heart of EIGRP is the Diffusing Update Algorithm (DUAL), the decision process for all route computations. DUAL uses distance information to select efficient, loop-free paths and selects the best route for insertion into the routing table, as well as a feasible successor. This feasible successor is used if the primary route becomes unavailable, thus avoiding a complete recalculation of the algorithm in the event of a link failure, and hence lowering convergence time. EIGRP also introduced a Reliable Transport Protocol to ensure guaranteed, ordered delivery of routing updates, rather than relying on broadcasts.
A key design feature of EIGRP is that it will support routing for protocols other than IP. EIGRP also supports routing for IPX and AppleTalk network protocols. This has some advantages in a mixed networking environment in that only one routing protocol need be configured.
EIGRP supports variable-length subnet masks (VLSM), which increase flexibility for the use of netmasks in internetwork design. Previously we discussed why an internetwork could have only one value of netmask assigned, as neither RIP nor IGRP carried netmask information in routing updates. EIGRP does carry this information.
The functionality afforded by VLSM is most useful when an organization has a limited address space assigned by the InterNIC, and has to supply these addresses to several sites of varying size (see Fig. 4-11).
Figure 4-11: Internetwork in which VLSM is used
We can see that network 2 must support 50 usable addresses, and network 1 must support 10. If network 2 has the most hosts at one site on the internetwork, and we are using RIP or IGRP, we would have to assign a netmask of 255.255.255.192, which gives us 62 usable addresses on each subnet. This would waste 52 usable addresses on network 1. With EIGRP we can assign a netmask of 255.255.255.192 to the Ethernet 0 port of router 2 and a 255.255.255.240 to the Ethernet 0 port of router 1. EIGRP will transmit this netmask information in its updates and routing table integrity will be maintained.
Inside EIGRP
The first table we will look at is the neighbor table. Neighbors are discovered when they send a hello packet to a neighboring router. All hello packets are used to form the neighbor table, which is where routers keep information regarding the state of adjacent neighbors. Hello packets include a HoldTime, the amount of time in which a router receiving the hello packet will treat the sender of the hello as reachable. If another hello is not received within the HoldTime, DUAL is informed of the change of state. This table also keeps track of Reliable Transport Protocol (RTP) sequence numbers and estimates an appropriate time for retransmission requests for RTP packets.
EIGRP has a topology table that contains all destinations advertised by neighboring routers, along with the metric value for each destination. This topology table forms the basis for input to the DUAL calculations. The output of DUAL feeds into the routing table. A topology table entry for a destination can be in one of two states, either active or passive (the normal condition). A route only becomes active when an event occurs to cause a route recalculation due to some router or link becoming unavailable.
An entry in the topology table gets moved to the routing table when what is known as a feasible successor has been found. A feasible successor can be thought of as the best alternative path to a destination, if the best path fails. If there is a feasible successor for an entry in the topology table, and the neighbor chosen for this entry stops sending hello packets, the entry will not go active in the topology table. Having a feasible successor therefore is a good thing, because it avoids a full recalculation of the DUAL algorithm in the event of a network failure.
EIGRP is a proprietary Cisco technology, and therefore has good compatibility with IGRP. EIGRP updates are not backwardly compatible with IGRP updates, but there is an automatic redistribution scheme for interoperability of IGRP and EIGRP routes, and there exist directly translatable metrics between IGRP and EIGRP. (Redistribution will be discussed later in this chapter.)
Configuring EIGRP
The following is a typical configuration to define EIGRP as a routing protocol on a Cisco router:
router eigrp 13
network 170.4.0.0
network 200.10.1.0
The first line defines EIGRP as a process on the router for autonomous system 13. The second and third lines identify the directly attached networks that participate in the routing process.
EIGRP supports unequal-cost load balancing by the variance command in a fashion similar to that discussed for IGRP; in fact, the same command is used to specify the variance multiplier.
One area of configuration that differs between IGRP and EIGRP is the option to disable route summarization. Route summarization is explained below.
In Fig. 4-12 we have two companies, Company A and Company B. Let's say Company A has to provide an information service to two locations of Company B. Company B uses the 170.2.0.0 network, with a netmask of 255.255.255.0, which means it can provide 254 subnets, each capable of supporting 254 hosts with this one network number. Company A uses the 150.1.0.0 network, with subnet masks applied, so that this one network number can be used to connect many client sites to its internetwork.
Figure 4-12: Internetwork in which route summarization causes a problem
If both companies are using EIGRP with a default configuration, the internetwork addressing shown in Fig. 4-12 will not work properly. The reason for that is that whole network numbers are summarized at their boundary. This means that the routers in the 150.1.0.0 internetwork do not see the 170.2.0.0 subnets, but rather see only an advertisement for the entire route. With IGRP or RIP, nothing can be done to change this, but with EIGRP there is an option we can use.
This can be a difficult concept for network administrators who have been accustomed to using netmasks on one network number and expect separate subnets to be treated as separate networks.
EIGRP provides the option to turn off route summerization, which means that network 150.1.0.0 will be able to accommodate more than one connection to the 170.2.0.0 network.
To help understand why this is so, let's consider the decision-making process a router within the 150.1.0.0 network goes through when it has to forward a packet to, say, 170.2.4.4 as an example. The first thing the router will do is determine the whole network number that the destination is on, which is 170.2.0.0. It will then look for an entry in its routing table for that network. The routing table will keep track of individual subnets within the 150.1.0.0 network, but does not do the same for external networks by default. This is so because it will not receive subnet information for the 170.2.0.0 network from the routers at the Company B locations.
In the case of Fig. 4-12, there will be two equal-cost routes in Company A's router to 170.2.0.0, so traffic will be split between them, irrespective of which subnet is the packet's real destination.
With EIGRP, route summarization can be disabled; the routing tables in 150.1.0.0 routers will maintain the two entries for the two subnets in the 170.2.0.0 network and be able to properly deliver packets to both subnets. The way this is done is to disable route summarization for all EIGRP processes. This means that router 1 and router 2 in Fig. 4-12 will send subnet information regarding the 170.2.0.0 network to the routers in Company A's routers.
To do this, enter the following EIGRP configuration commands for all routers involved in accessing subnets on multiple network numbers:
Router1(config)#router eigrp 11
Router1(config-router)no auto-summary

 


 
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