The link between Routers A and B is only a 64-kbps link, but the other two links are T1 links. So the better route from Router B to subnet 10.1.1.0 arguably is through Router C. That route requires an extra hop through a router, but it uses two T1 links instead of one relatively slow 64-kbps link.

RIP uses hop count only for the metric, so regardless of the bandwidth command settings, RIP on Router B chooses the route over the slow link directly to Router A. IGRP, with the default bandwidth settings in the middle part of the figure, also chooses the route directly through Router A. However, as shown in the bottom part of the figure, after the bandwidth commands configure the correct values, IGRP chooses the better route to subnet 10.1.1.0 through Router C.

Examination of RIP and IGRP debug and show Commands

This section on basic RIP and IGRP configuration closes with one more sample network that is first configured with RIP and then is configured with IGRP. Advanced distance vector protocol concepts, such as split horizon and route poisoning, become more obvious when you look at these examples. RIP and IGRP implement split horizon and route poisoning. You can better understand them by examining the upcoming debug messages.

168 Chapter 5: RIP, IGRP, and Static Route Concepts and Configuration

First, Figure 5-11 and Example 5-7 show a stable RIP network with split-horizon rules that affect the RIP updates. The numbered arrows in the figure represent routing updates. The comments in Example 5-7 refer to those same numbers to help you correlate the figure and the example. Example 5-7 shows Ethernet 0 on Yosemite being shut down and Yosemite advertising an infinite-distance route to 10.1.2.0. The route is poisoned, as shown in the debug ip rip output at the end of the example. Example 5-8 shows the corresponding RIP configuration in Yosemite.

Example 5-7 RIP Configuration and Debugs on Albuquerque

interface ethernet 0

ip addr 10.1.1.251 255.255.255.0 interface serial 0

ip addr 10.1.4.251 255.255.255.0 interface serial 1

ip addr 10.1.6.251 255.255.255.0

!

router rip network 10.0.0.0

Albuquerque#debug ip rip

RIP: received v1 update from 10.1.6.253 on Serial1 10.1.3.0 in 1 hops

10.1.2.0 in 2 hops 10.1.5.0 in 1 hops

RIP: sending v1 update to 255.255.255.255 via Serial0 (10.1.4.251)

170 Chapter 5: RIP, IGRP, and Static Route Concepts and Configuration

Example 5-8 RIP Configuration on Yosemite

interface ethernet 0

ip addr 10.1.2.252 255.255.255.0 interface serial 0

ip addr 10.1.4.252 255.255.255.0 interface serial 1

ip addr 10.1.5.252 255.255.255.0

router rip network 10.0.0.0

First, examine the configuration on Albuquerque (Example 5-7) and Yosemite (Example 5-8). Because all interfaces on each router are part of network 10.0.0.0, RIP needs only a single network command on each router, so the configuration is relatively easy.

For the rest of the explanation, refer to the phrases “POINT NUMBER X” in Example 5-7. The following list describes what happens at each point in the process:

■POINT NUMBER 1—Albuquerque sends an update out Serial0, obeying split-horizon rules. Notice that 10.1.2.0, Yosemite’s Ethernet subnet, is not in the update sent out Albuquerque’s S0 interface.

■POINT NUMBER 2—This point begins right after Yosemite’s E0 is shut down, simulating a failure. Albuquerque receives an update from Yosemite, entering Albuquerque’s S0 interface. The route to 10.1.2.0 has an infinite metric, which in this case is 16.

■POINT NUMBER 3—Albuquerque formerly did not mention subnet 10.1.2.0 because of split-horizon rules (point 1). The update at point 3 includes a poisoned route for 10.1.2.0 with metric 16. This is an example of split horizon with poison reverse.

■POINT NUMBER 4—Albuquerque receives an update in S1 from Seville. The update includes a metric 16 (infinite) route to 10.1.2.0. Seville does not suspend any splithorizon rules to send this route, because it saw the advertisement of that route earlier, so this is a simple case of route poisoning.

Example 5-9 shows the steps needed to migrate to IGRP. It also lists some debug and show commands. It shows the configuration added to each of the three routers shown in Figure 5-11 to migrate to IGRP. The logic of the network commands works just like with RIP. The output of the show and debug commands provides some insight into the differences between RIP and IGRP.

NOTE The following configuration commands are used on all three routers.

Configuring RIP and IGRP 171

Example 5-9 Migrating to IGRP with Sample show and debug Commands

no router rip router igrp 5 network 10.0.0.0

Albuquerque#show 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, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR

P - periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 6 subnets

I10.1.3.0 [100/8539] via 10.1.6.253, 00:00:28, Serial1

I10.1.2.0 [100/8539] via 10.1.4.252, 00:00:18, Serial0 C 10.1.1.0 is directly connected, Ethernet0

C 10.1.6.0 is directly connected, Serial1

I10.1.5.0 [100/10476] via 10.1.4.252, 00:00:18, Serial0

[100/10476] via 10.1.6.253, 00:00:29, Serial1 C 10.1.4.0 is directly connected, Serial0 Albuquerque#debug ip igrp transactions

IGRP protocol debugging is on

Albuquerque#

07:43:40: IGRP: sending update to 255.255.255.255 via Serial0 (10.1.4.251)

07:43:40: IGRP: sending update to 255.255.255.255 via Serial1 (10.1.6.251)

07:43:40: IGRP: sending update to 255.255.255.255 via Ethernet0 (10.1.1.251)

07:43:59: IGRP: received update from 10.1.6.253 on Serial1 07:43:59: subnet 10.1.3.0, metric 8539 (neighbor 688) 07:43:59: subnet 10.1.5.0, metric 10476 (neighbor 8476) 07:44:18: IGRP: received update from 10.1.4.252 on Serial0 07:44:18: subnet 10.1.2.0, metric 8539 (neighbor 688) 07:44:18: subnet 10.1.5.0, metric 10476 (neighbor 8476) Albuquerque#no debug all

continues

172 Chapter 5: RIP, IGRP, and Static Route Concepts and Configuration

Example 5-9 Migrating to IGRP with Sample show and debug Commands (Continued)

All possible debugging has been turned off

Albuquerque#

Albuquerque#debug ip igrp events

IGRP event debugging is on

All possible debugging has been turned off

Albuquerque#show ip protocol

Routing Protocol is “igrp 5”

Sending updates every 90 seconds, next due in 34 seconds

Invalid after 270 seconds, hold down 280, flushed after 630

Outgoing update filter list for all interfaces is

Incoming update filter list for all interfaces is

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

IGRP maximum hopcount 100

Distance: (default is 100)

You can migrate from RIP to IGRP in this case with only three configuration commands per router. As highlighted in Example 5-9, the no router rip command removes all RIP configuration on the router, including any network subcommands. The three routers each