Content

| 5.4 |  | Troubleshooting RIP |
| 5.4.5 |  | Invalid source address |

When RIP tells the routing table to install the route, it performs a source-validity check. If the source is not on the same subnet as the local interface, RIP ignores the update and does not install the routes coming from this source address in the routing table.

> Note: This same issue exists with IGRP. In the case of EIGRP and OSPF, routers will not be able to form neighbor relationships if they are not on the same subnet. OSPF performs the subnet number and mask check on all media except point-to-point and virtual links.

In Figure , the R1 Serial 0 interface is unnumbered to Loopback 0. The Router R2 serial interface is numbered. When router R2 receives a RIP update from R1, the source address is considered invalid because the source address is not on the same subnet as the R2 serial interface.

Debugs and Verification
Figure shows the configuration of both router R2 and R1. In this configuration, the R1 Serial 0 interface is unnumbered to Loopback 0. The R2 Serial interface is numbered.

The debug ip rip output shows that R2 is ignoring the RIP update from R1 because of a source validity check. The RIP update coming from R1 is not on the same subnet, so R2 will not install any routes in the routing table from R1.

Solution
When one side is numbered and the other side is unnumbered, this check must be disabled. This is usually the case in a dialup situation when remotes are dialing into an access router. The access routers dialup interface is unnumbered, and all remote routers get an IP address assigned on their dialup interfaces. This same solution can also apply when both sides of the link are unnumbered.

Figure shows the configuration change on router R2 to fix this problem.

Figure shows that after changing the configuration of R2, the route gets installed in the routing table.