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Chapter 5
IP Routing
Configuring IGRP is pretty straightforward and not much different from
configuring RIP. You do need to decide on an AS number before you con-
figure your routers. Remember that all routers in your internetwork must
use the same AS number if you want them to share routing information. In
the sample internetwork we've been using throughout this chapter, we'll use
AS 10 to configure the routers.
Okay, let's configure our internetwork with IGRP routing.
2621A
The AS number, as shown in the router output below, can be any number from
1 to 65535. A router can be a member of as many ASs as you need it to be.
2621A#config t
Enter configuration commands, one per line.
End with CNTL/Z.
2621A(config)#router igrp ?
<1-65535> Autonomous system number
2621A(config)#router igrp 10
2621A(config-router)#netw 172.16.0.0
2621A(config-router)#^Z
2621A#
The router igrp command turns IGRP routing on in the router. As with
RIP, you still need to add the network number you want to advertise. IGRP
uses classful routing, which means that subnet mask information is not sent
with the routing protocol updates.
Understand that if you did type in network 172.16.10.0, the router would accept
it and then change the configuration to a classful entry of 172.16.0.0. However,
the exam will not be so forgiving and will mark your answer wrong if you
type the wrong network number. I cannot stress this enough: think classful!
2501A
To configure the 2501A router, all you need to do is turn on IGRP routing
using AS 10 and then add the network number, as shown next.
2501A#config t
Enter configuration commands, one per line.
End with CNTL/Z.
2501A(config)#router igrp 10
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