Configuring IGRP

The configuration of IGRP is only slightly different than that of RIP. The major difference is that IGRP routers are made part of what is known as an autonomous system (AS), a grouping that defines routers that should exchange routing tables. For example, if a router is made part of AS 100, it will only exchange IGRP routing information with other routers that are part of IGRP AS 100. Many IGRP autonomous systems can be defined within an internetwork, allowing you a more granular level of control over which routers exchange routing table information with one another.

Similar to RIP, you must also specify the networks to be included in IGRP updates. In this configuration, we are going to add IGRP AS 100 to our existing RIP network. This will also help to show what happens when two routing protocols are used on the same network. To review the network, look back at Figure 16. To add IGRP routing, use the router igrp command, followed by the autonomous system number.

RouterA#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#router igrp 100

Of course, IGRP also needs to be enabled on Router B:

RouterB#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterB(config)#router igrp 100

A look at the routing table on Router A shows that that network has been added, but via IGRP instead of RIP.

RouterA#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, * - candidate default
U - per-user static route, o - ODR
Gateway of last resort is not set is subnetted, 3 subnets
C is directly connected, Ethernet0
I [100/8576] via, 00:00:01, Serial0
C is directly connected, Serial0

Notice that the listing for network begins with an I, which designates that the route was found using IGRP. The RIP route is no longer available. How does this happen? This is due to that fact that IGRP has a lower administrative distance than RIP. In cases where a route is found by a protocol with a lower administrative distance, the routing table will only show the more trustworthy route. In this case, both RIP and IGRP are announcing the same route, but IGRP’s administrative distance of 100 beats RIP’s administrative distance of 120. The administrative distance and composite metric are both listed in the entry as well – [100/8576], where 8576 is the composite metric.

Like RIP, IGRP is also capable of load balancing over up to 6 links, including links with uneven costs.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.