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Chapter 8 - Classless Routing Protocols

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

Chapter 8: Classless Routing Protocols
Overview
To be able to take advantage of VLSM and discontiguous network addressing we must use a routing protocol that will carry a subnet mask. Such a routing protocol is called classless.
Examples of classless routing protocols include Open Shortest Path First (OSPF), RIPv2, and Enhanced Interior Gateway Routing Protocol (EIGRP).
Let us take another look at the ARI scenario from Chapter 7 (Figure 8.1).
Given a single IP address and a classful routing protocol, there would be no way of configuring all hosts of ARI.
As a result, a classless routing protocol must be used.
Figure 8.1  ARI Topology
Figure 8.2 shows a portion of an OSPF debug. This output illustrates how router SANFRAN received updates relating to the serial connections and mask to TAMPA and DALLAS.
SANFRAN# debug ip ospf events
OSPF: Add Network Route to 192.16.15.8 Mask /30. Metric: 65, Next Hop: 192.16.15.5
OSPF: Add Network Route to 192.16.15.12 Mask /30. Metric: 65, Next Hop: 192.16.15.5
Figure 8.2  OSPF Debug
Figures 8.3 and 8.4 show the OSPF and the EIGRP routing tables respectively, of JAX.
Router JAX “learned” all of the subnets of the 198.10.11.0 network.
JAX#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
  199.10.11.0/24 is variably subnetted, 7 subnets, 4 masks
O    199.10.11.64/27 [110/879] via 199.10.11.13, 00:01:03, Serial2
C    199.10.11.4/30 is directly connected, Serial0
C    199.10.11.12/30 is directly connected, Serial2
C    199.10.11.8/30 is directly connected, Serial1
C    199.10.11.128/26 is directly connected, Etherent0
O    199.10.11.32/28 [110/74] via 199.10.11.5, 00:01:03, Serial0
O    199.10.11.48/28 [110/74] via 199.10.11.9, 00:01:03, Serial1
Figure 8.3  OSPF Routing Table
JAX#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
  199.10.11.0/24 is variably subnetted, 7 subnets, 4 masks
D    199.10.11.64/27 [90/22798336] via 199.10.11.13, 00:00:40, Serial2
C    199.10.11.4/30 is directly connected, Serial0
C    199.10.11.12/30 is directly connected, Serial2
C    199.10.11.8/30 is directly connected, Serial1
C    199.10.11.128/26 is directly connected, Ethernet0
D    199.10.11.32/28 [90/2195456] via 199.10.11.5, 00:00:41, Serial0
D    199.10.11.48/28 [90/2195456] via 199.10.11.9, 00:00:40, Serial1
Figure 8.4  EIGRP Routing Table
Notice that router JAX knows where each of the subnets are located.
Another advantage of using a classless routing protocol is that we can use a discontiguous addressing scheme.
This fact is illustrated in Figures 8.5 and Figure 8.6.
Figure 8.5  Discontiguous Topology
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
   10.0.0.0/24 is subnetted, 2 subnets
O  10.0.1.0 [110/74] via 172.16.1.1, 00:02:41, Serial1
O  10.0.4.0 [110/74] via 172.16.2.1, 00:02:41, Serial0
Figure 8.6  OSPF Routing Table
Even though network 172.16.0.0 is in between network 10.0.0.0 Router_A can still distinguish which interface to use to get to either subnet of network 10.0.0.0.
A common question is “Which is the best routing protocol, OSPF or EIGRP?”
Keep in mind that OSPF is an open routing protocol, meaning that all vendors support OSPF. EIGRP on the other hand is a proprietary protocol belonging to Cisco. You must have all networking devices running the Cisco IOS to support EIGRP.
We will configure OSPF and EIGRP in later chapters.

 


 
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