Network Tip No. 31: IGRP Routing Commands

IGRP Routing: Mandatory Commands

Router(config)#router igrp as-number	Enables IGRP routing process. The autonomous system number (AS-number) used in the IGRP routing process must match all other routers that are going to share routing updates in order for communication to take place
Router(config-router)#network w.x.y.z	w.x.y.z is the network number of the directly connected network you want to advertise

Note:

You need to advertise only the classful network number, not a subnet:

Router(config-router)#network 172.17.0.0 

not

Router(config-router)#network 172.17.10.0 

If you advertise a subnet, you will not receive an error message, because the router will automatically convert the subnet to the classful network address.