Threaded Case Study


IGRP is a distance-vector interior gateway protocol (IGP). Distance-vector routing protocols call for each router to send all or a portion of its routing table in a routing-update message at regular intervals to each of its neighboring routers. By default, it will exchange its routing tables with its directly connected neighbors every 90 seconds. Flash updates, triggered by topology changes, are also sent. Any routing protocol will use up some of the bandwidth otherwise available for data, and IGRP, since it has various metrics with which to monitor and adjust network traffic, has a fair amount of data in its updates.

Advantage of IGRP

An advantage of using IGRP instead of another distance-vector protocol, RIP, is that IGRP is not bound to use hop count as a metric for path determination. The two metrics we will use are Bandwidth and Delay, which are two reasonable metrics that will suite our demands and not slow down the routing decision time too much. The router sends and receives IGRP updates, routing tables, to its neighbor router in the school district every 90 seconds. This should not overload the network.

Stability Features

IGRP provides a number of features that are designed to enhance its stability. These include hold-downs, split horizons, and poison-reverse updates.

Hold-downs: are used to prevent regular update messages from inappropriately reinstating a route that might have gone bad.

Split horizons: information cannot be sent back in the direction from which that information was received.

Poison-reverse updates: are necessary to defeat larger routing loops. Increases in routing metrics generally indicate routing loops. Poison-reverse updates are then sent to remove the route and place it in hold-down.


IGRP maintains a number of timers and variables containing time intervals.
Update timer: specifies how frequently routing update messages should be sent. The IGRP default for this variable is 90 seconds.

Invalid timer: specifies how long a router should wait, in the absence of routing-update messages about a specific route before declaring that route invalid. The IGRP default for this variable is three times the update period.

Holdown-timer: specifies the hold-down period. The IGRP default for this variable is three times the update timer period plus 10 seconds.

Flush timer: indicates how much time should pass before a route should be flushed from the routing table. The IGRP default is seven times the routing update period.

The networks that will be advertised by our site router are:

IGRP Commands

Autonomous system number: 13

Router sequence:
router# config t
router (config)# router igrp 13
router (config-router)# network
router (config-router)# network
router (config-router)# timers basic 100 200 300 400

IGRP broadcasts router updates to all neighboring routers every 90 seconds by default. This has been changed to a 120 second delay due to the invariance in network addressing for this particular network. If a link goes down, a route is declared invalid (default time 270 seconds, changed to 360 seconds). After the route becomes invalid, the router enters holddown, 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 seconds, changed to 370 seconds). The route is removed from the routing table when the flush timer expires (default time 630 seconds, changed to 850 seconds). The timers have been changed to alleviate bandwidth strains due to broadcast storms.