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Chapter 6 - Classful Routing Protocols and Network Topology

Cisco & IP Addressing
Louis D. Rossi, Louis R. Rossi and Thomas Rossi
  Copyright © 1999 The McGraw-Hill Companies, Inc.

Chapter 6: Classful Routing Protocols & Network Topology
Overview
Classful routing protocols do not carry subnet mask information across different major networks. A classful router always assumes that the same subnet mask is used everywhere for the same major network. For example, if we are using the major network address of 10.0.0.0 with a subnet mask of 255.255.255.0 it is always assumed that the network 10.0.0.0 will always carry the mask of 255.255.255.0.
A Class C address has a default mask of 255.255.255.0; a classful routing protocol will always assume this mask is used for the same major network address when the update is received from another major network address.
A classful routing protocol does not communicate a subnet mask other than the default mask for that class of address, hence the name classful. A classless routing protocol will communicate the subnet mask. Chapter 8 will discuss this in greater detail.
There are two important rules to follow when using a classful routing protocol:
  1. The network must be addressed in a contiguous manner.
  2. Use the same mask for the same major network.
The word contiguous implies that the same major network address is carried continuously across the topology; refer to Figure 6.1. The address of 10.0.0.0 and the mask of 255.255.255.0 is used for every network.
Figure 6.1  Contiguous Addressing
As shown in Figure 6.2 Router_A learns of the 10.0.4.0 subnet.
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
  U - per-user static route, o – ODR
Gateway of last resort is not set
  10.0.0.0/24 is subnetted, 3 subnets
C    10.0.3.0 is directly connected, Serial0/0
C    10.0.1.0 is directly connected, Ethernet0/0
I    10.0.4.0 [100/8576] via 10.0.3.1, 00:01:11, Serial0/0
Figure 6.2  Router_A Routing Table
Router_A “learns” that the subnet 10.0.4.0 is 1 hop away.
Figures 6.3 and 6.4 illustrate that IP RIP and IGRP, both classful routing protocols, will advertise subnets across the SAME major network!
Refer to the bold lines below:
Router_A#debug ip igrp transactions
IGRP: sending update to 255.255.255.255 via Ethernet0/0 (10.0.1.1)
  subnet 10.0.3.0, metric=8476
  subnet 10.0.4.0, metric=8576
IGRP: sending update to 255.255.255.255 via Serial0/0 (10.0.3.2)
  subnet 10.0.1.0, metric=1100
IGRP: received update from 10.0.3.1 on Serial0/0
  subnet 10.0.4.0, metric 8576 (neighbor 1100)
Figure 6.3  IGRP Debug
Router_A#debug ip rip
RIP protocol debugging is on
Router_A#
RIP: sending v1 update to 255.255.255.255 via Ethernet0/0 (10.0.1.1)
  subnet  10.0.3.0, metric 1
RIP: sending v1 update to 255.255.255.255 via Serial0/0 (10.0.3.2)
  subnet  10.0.1.0, metric 1
RIP: received v1 update from 10.0.3.1 on Serial0/0
     10.0.4.0 in 1 hops
Figure 6.4  IP RIP Debug
In a discontiguous environment a major network address will be “broken” up by another major network address. Refer to Figure 6.5.
Figure 6.5  Discontiguous Addressing-1
Figure 6.5 shows the “172” network in the middle of the “10” network.
This type of topology presents a problem with a classful routing protocol. As illustrated in Figure 6.6 Router_A does not learn of network 10.0.4.0.
Router_A#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
  U - per-user static route, o - ODR
Gateway of last resort is not set
  172.16.0.0/24 is subnetted, 1 subnets
C    172.16.1.0 is directly connected, Serial0/0
  10.0.0.0/24 is subnetted, 1 subnets
C    10.0.1.0 is directly connected, Ethernet0/0
Figure 6.6  Router_A Routing Table
Router_A does receive updates relating to network 10.0.0.0, but not to network 10.0.4.0. Router_A will send all packets with a destination address of 10.0.0.0 out of the Ethernet interface.
Subnets are not carried across different major networks.
These updates are ignored because the metric is higher than the directly connected network of 10.0.0.0.
Figures 6.7 and 6.8 show that Router_A receives an update relating to network 10.0.0.0 but assumes the default mask and discards the update as not being the best way to get to the destination.
Router_A#debug ip igrp tr
IGRP protocol debugging is on
Router_A#
IGRP: received update from 172.16.1.2 on Serial0/0
     network 10.0.0.0, metric 8576 (neighbor 1100)
IGRP: sending update to 255.255.255.255 via Ethernet0/0 (10.0.1.1).
     network 172.16.0.0, metric=8476
IGRP: sending update to 255.255.255.255 via Serial0/0 (172.16.1.1)
     network 10.0.0.0, metric=1100
Figure 6.7  IGRP Debug
The composite metric of 8576 is higher than the metric of a directly connected network. Therefore Router_A will discard the routing update pertaining to network 10.0.0.0 that is received from the serial interface.
Router_A#debug ip rip
RIP protocol debugging is on
Router_A#
RIP: received v1 update from 172.16.1.2 on Serial0/0
     10.0.0.0 in 1 hops
RIP: sending v1 update to 255.255.255.255 via Ethernet0/0 (10.0.1.1)
     network 172.16.0.0, metric 1
RIP: sending v1 update to 255.255.255.255 via Serial0/0 (172.16.1.1)
     network 10.0.0.0, metric 1
Figure 6.8  IP RIP Debug
IP RIP uses hop count as its metric therefore, 1 hop away is a higher metric than a directly connected network. The RIP update received from the serial link will be discarded.
With a discontiguous network and a classful routing protocol the routers do not see a true picture of the network topology.
Because we have a discontiguous network and a classful routing protocol Router_A gets a distorted view of the network topology. This distorted view is represented in Figure 6.9
Figure 6.9  Distorted View of the Network Topology
Figure 6.9 illustrates the view that Router_A has two ways to get to network 10.0.1.0. Router_A will always send a packet destined for network 10.0.0.0 to the Ethernet 0 interface.
Now let us examine another discontiguous network, but we will throw in another router. Keep in mind we have broken the rule of keeping our addressing scheme contiguous, which means we should see some problems with the routing tables. Refer to Figure 6.10.
Figure 6.10  Discontiguous Addressing-2
Router_C#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
  U - per-user static route, o - ODR
Gateway of last resort is not set
  172.16.0.0/24 is subnetted, 2 subnets
C   172.16.1.0 is directly connected, Serial1
C   172.16.2.0 is directly connected, Serial0
I   10.0.0.0/8 [100/8576] via 172.16.1.1, 00:00:09, Serial1
       [100/8576] via 172.16.2.1, 00:00:19, Serial0
Figure 6.11  Router_C Routing Table
Referring to the bold lines in Figure 6.11, Router_C “thinks” it can get to network 10.0.0.0 in two different ways, through Serial 1 or Serial 0.
Router_C has no way to distinguish between the different subnets of “network 10” because the subnets of a classful routing protocol are not carried across different major networks.
Since Router_C thinks it has two ways to get to network 10.0.0.0 it will load balance the traffic over the two serial interfaces. One packet will go out Serial 0, the next will use Serial 1, then back to Serial 0, etc.
Figure 6.12 represents pings from Router_C to network 10.0.4.1
Router_C#ping 10.0.4.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.4.1, timeout is 2 seconds:
U!.!U
Success rate is 40 percent (2/5), round-trip min/avg/max = 4/4/4 ms
Router_C#ping 10.0.4.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.4.1, timeout is 2 seconds:
!U!.!
Success rate is 60 percent (3/5), round-trip min/avg/max = 4/4/4 ms
Figure 6.12  Pings from Router_C
10.0.4.1 will be located with every other ping probe, resulting in a 50% average success rate.
Now we will break another rule and NOT use the same subnet mask with a classful routing protocol.
Figure 6.13 illustrates a mask of 255.255.255.0 for the serial link and a mask of 255.255.0.0 for the Ethernet links
Figure 6.13  Different Masks for the Same Major Network
Router_A now has no way of knowing the true topology of this network. We could speculate on what will happen when Router_A attempts to route a packet, but what is the point!
The bottom line is that this IP addressing design is wrong and needs to be changed.
The routing table shows Router_A informing us that network 10.0.0.0 is variably subnetted. This is not a good thing!
Router_A#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
  D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
  N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
  E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
  i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
  U - per-user static route, o – ODR
Gateway of last resort is not set
  10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C    10.2.0.0/16 is directly connected, Ethernet0/0
C    10.0.3.0/24 is directly connected, Serial0/0
Figure 6.14  Router_A Routing Table
A classful routing protocol presents the network designer with some severe restrictions.
Not only does the network need to be configured in a contiguous manner, but there could be a waste of IP addresses.
We will discuss these restrictions and some solutions in later chapters.
Figure 6.15 is a summary outline of classful and classless routing protocols.
A. Classful Routing Protocols
   1. IP RIPv1
   2. IGRP
   3. IPX RIP (Novell)
   4. RTMP (Apple)
B. Classless Routing Protocols
   1. EIGRP
   2. OSPF
   3. IS-IS
   4. NLSP (Novell)
   5. IP RIPv2
Figure 6.15  Classful and Classless Routing Protocols

 


 
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