Interior Gateway Routing Protocol (IGRP) is an advanced distance vector routing protocol developed by Cisco in the middle 1980s. IGRP has several features that differentiate it from other distance vector routing protocols such as RIP.

Use the router igrp autonomous-system command to enable the IGRP routing process:

R1(config)#router igrp 100

Use the router configuration network network-number command to enable interfaces to participate in the IGRP update process:

R1(config-router)#network 172.30.0.0
R1(config-router)#network 192.168.3.0

Verify IGRP configuration with the show running-configuration and show ip protocols commands:

R1#show ip protocols

Verify IGRP operation with the show ip route command:

R1#show ip route

If IGRP does not appear to be working correctly, check the following:

• Layer 1 or Layer 2 connectivity issues exist.
• Autonomous system numbers on IGRP routers are mismatched.
• Network statements are missing or incorrectly assigned.
• The outgoing interface is down.
• The advertised network interface is down.

To view IGRP debugging information, use the following commands:

• debug ip igrp transactions [host ip address] to view IGRP transaction information
• debug ip igrp events [host ip address] to view routing update information

To turn off debugging, use the no debug ip igrp command.

If a network becomes inaccessible, routers running IGRP send triggered updates to neighbors to inform them. A neighbor router will then respond with poison reverse updates and keep the suspect network in a holddown state for 280 seconds.   

Web Links IGRP http://www.cisco.com/en/US/tech/ tk365/tk352/technologies_tech_note 09186a0080093f0b.shtml