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Chapter 5

IP Routing

Interior Gateway Routing Protocol (IGRP)

nterior Gateway Routing Protocol (IGRP) is a Cisco-proprietary

distance-vector routing protocol. This means that all your routers must
be Cisco routers to use IGRP in your network. Cisco created this routing
protocol to overcome the problems associated with RIP.

IGRP has a maximum hop count of 255 with a default of 100. This is

helpful in larger networks and solves the problem of 15 hops being the max-
imum possible in a RIP network.

IGRP also uses a different metric from RIP. IGRP uses bandwidth and

delay of the line by default as a metric for determining the best route to an
internetwork. This is called a composite metric. Reliability, load, and max-
imum transmission unit (MTU) can also be used, although they are not used
by default.

The main difference between RIP and IGRP configuration is that when you

configure IGRP, you supply the autonomous system number. All routers must

use the same number in order to share routing table information.

IGRP Timers

To control performance, IGRP includes the following timers with default
settings:

Update timers These specify how frequently routing-update messages
should be sent. The default is 90 seconds.

You can do what is called redistribution, which is basically translating from

one type of routing protocol to another. This means that you can support

that old network using RIP but use Enhanced IGRP, for example, on the rest

of your network. This will stop RIP routes from being sent all through the

network and using up precious bandwidth.

Redistribution is covered in the Sybex CCNP: Routing Study Guide.

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