The way you enable IGRP on a Cisco router is similar to the way you enable RIP, except you specify IGRP as the protocol and add an autonomous system number. For example:

RouterA(config)#router igrp 10 (10 is the AS number)
RouterA(config-router)#network 192.168.0.0
RouterA(config-router)#network 192.168.1.0
RouterA(config-router)#network 192.168.2.0

The four timers IGRP uses to regulate its performance are as follows:

1. Route update timer Time between router updates The default is 90 seconds.
2. Route invalid timer Time that must expire before a route becomes invalid . The default is 270 seconds.
3. Route hold-down timer If a destination becomes unreachable, or if the next-hop router increases the metric recording in the routing table, the router goes into holddown for 280 seconds.
4. Route flush timer[md]Time from when a route becomes invalid to when it is removed from the routing table. The default is 630 seconds.

A broadcast domain defines a group of devices that receive each others' broadcast messages. As with collisions, the more broadcasts that occur on the network, the slower your network will be. This is because every device that receives a broadcast must process it to see if the broadcast is intended for it.

Switches and bridges are used to break up collision domains. They create more collision domains and fewer collisions. Routers are used to break up broadcast domains. They create more broadcast domains and smaller broadcast areas.

Each layer of the OSI model can communicate only with the layer above it, below it, and parallel to it (a peer layer). For example, the presentation layer can communicate with only the application layer, session layer, and presentation layer on the machine it is communicating with. These layers communicate with each other using protocol data units (PDUs). These PDUs control information that is added to the user data at each layer of the model. This information resides in fields called headers (the front of the data field) and trailers (the end of the data field).