Cisco Systems 78-11741-02 manual Igrp Configuration Task List, Igrp Updates, IPC-214

Configuring IGRP

IGRP Configuration Task List

Figure 36 Interior, System, and Exterior Routes

Autonomous system 1

	Subnet B	System
Interior	Router	Router
	Subnet A

IGRP Updates

Autonomous system 2

Exterior

Router

S1019a

By default, a router running IGRP sends an update broadcast every 90 seconds. It declares a route inaccessible if it does not receive an update from the first router in the route within three update periods (270 seconds). After seven update periods (630 seconds), the Cisco IOS software removes the route from the routing table.

IGRP uses flash update and poison reverse updates to speed up the convergence of the routing algorithm. Flash update is the sending of an update sooner than the standard periodic update interval of notifying other routers of a metric change. Poison reverse updates are intended to defeat larger routing loops caused by increases in routing metrics. The poison reverse updates are sent to remove a route and place it in holddown, which keeps new routing information from being used for a certain period of time.

IGRP Configuration Task List

To configure IGRP, perform the tasks described in the following sections. The tasks in the first section are required; the tasks in the remaining sections are optional:

•Creating the IGRP Routing Process (Required)

•Applying Offsets to Routing Metrics (Optional)

•Allowing Unicast Updates for IGRP (Optional)

•Defining Unequal-Cost Load Balancing (Optional)

•Controlling Traffic Distribution (Optional)

•Adjusting the IGRP Metric Weights (Optional)

•Adjusting Timers (Optional)

Cisco IOS IP Configuration Guide

IPC-214