Static Routing and Distance Vector Routing Protocols - CCIE Routing and Switching Exam Certification Guide - page 337

IGRP

IGRP was developed by Cisco Systems to overcome the limitations of RIPv1. IGRP is a
distance vector routing protocol that uses bandwidth (and other metrics) instead of hop count
as the metric. IGRP is not limited to the 15-hop limit of RIP. Also, faster links that might have
a longer hop count are selected as the best path over small hop count routes that might be using
slower links.(Note: According to a recent announcement posted on Cisco System's web site, the
351-001 written exam does not test candidates on IGRP. Because the blueprint for the 350-001
exam includes mention of IGRP, the information is included here.)

IGRP is a classful protocol and cannot implement VLSMs or CIDR. IGRP also summarizes at
network boundaries. As in RIP, IGRP implements split horizon with poison reverse, triggered
updates, and holddown timers for stability and loop prevention. Another benefit of IGRP is that
it can load balance over unequal-cost links. As a routing protocol developed by Cisco, IGRP is
available only on Cisco routers.

By default, IGRP loads balance traffic if there are several paths with equal-cost to the destination.
IGRP does unequal-cost load balancing if configured with the variance n command. IGRP
includes routes that are equal to or less than n times the minimum metric route to a destination.

As in RIP, IGRP also summarizes IP networks at network boundaries. Autosummarization can
be disabled with the no auto-summary command, as follows:

router igrp 100
no auto-summary

IGRP Timers
IGRP sends its routing table to its neighbors every 90 seconds. IGRP's update period of 90
seconds is a benefit over RIP, which can overpower bandwidth by sending updates every 30
seconds. IGRP also uses an invalid timer to mark a route as invalid after 270 seconds (3 times
the update timer). As with RIP, IGRP also uses a flush timer to remove a route from the routing
table; the default flush timer is set to 630 seconds (7 times the update period, over 10 minutes).

If a network goes down or the metric for the network increases, the route is placed in
holddown—no new changes are accepted for the route until the holddown timer expires. This
is done to prevent routing loops in the network. The default holddown timer is 280 seconds
(3 times the update timer plus 10 seconds).

Table 7-1 summarizes the default settings for IGRP timers.

Table 7-1

IGRP Timers

IGRP Timer Default Time

Update 90 seconds

Invalid 270 seconds

Holddown 280 seconds

Flush 630 seconds