Content

| 5.5 |  | Troubleshooting IGRP |
| 5.5.2 |  | AS mismatch |

IGRP updates carry their autonomous system (AS) number. When a receiver receives an IGRP update and the AS number of the sender does not match with its own AS number, IGRP ignores that update. As a result, no IGRP routes are installed in the routing table. Multiple IGRP processes can be run under different AS numbers. These processes are independent of each other.

> Note: The same is true with EIGRP, as the AS numbers must also match, or EIGRP routers will not form a neighbor relationship with its adjacent router.

Debugs and Verification
Figure shows the configuration of both routers R1 and R2. R1 is running IGRP AS 1, and R2 is running IGRP AS 2.

Figure shows the output of the debug ip igrp transaction and debug ip packet 100 detail commands on routers R1 and R2. IGRP is ignoring these updates because of the AS number mismatch. Unfortunately, the debug output does not show the mismatch message. However, it does show that IGRP is not displaying the update received message in the debugs.

Solution
In this example, the sender is sending AS 1 in the update. When R2 receives it, it ignores this update because R2 is running IGRP under AS 2. To fix this problem, change the IGRP configurations so that R1 and R2 both agree on one AS number.

Figure shows the new configuration on R2 that fixes the problem.

Figure shows that after fixing the AS mismatch problem, IGRP routes get installed in the routing table.