Washington School District has been asigned the autonomous network number 100. Mountain Sky is using IGRP for network To configure IGRP on the Mountain Sky router the following commands should be used:

Mtnsky# configure terminal

Mtnsky(config)# router igrp 100

Mtnsky(config-router)# network

Mtnsky(config-router)# timers basic 120 360 370 850

Mtnsky(config-router)# exit

Mtnsky(config)# exit


IGRP overview

IGRP broadcast's router updates to all neighboring routers every 90 seconds by default. If a link goes down a route is declared invalid (default time 270). After the route becomes invalid the router enters hold down and the route is advertised as unreachable but still used for forwarding packets. The router suppresses information regarding a better path until the holddown timer has expired (default time 280). The route is removed from the routing table when the flush timer expires default time 630).

You can adjust the way routers send updates by using the timers basic update invalid holddown flush command. To reset the timers to the default settings you would enter the no timers basic command. Raising the timers reduces the number of broadcasts that are sent causing less of a burden on the networks bandwidth. LAN's that do not require many changes could send fewer updates. Lowering the timers increases the speed at which a network converges and maintains an up to date overview of the LAN. This increases the amount of bandwidth the routers need to communicate slowing down the network.

IGRP will be used on the Mountain Sky router to maintain and advertise routing table updates to other routers in the school district. IGRP routing updates will be sent at the default specification of 90 seconds.  More frequent updates are not necessary because of the small size of this system.  Effect on bandwidth will be minimal due to the size of the network and by keeping the updates set at the default of 90 seconds.


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