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| 5.4 |  | Troubleshooting RIP |
| 5.4.6 |  | Flapping routes |

Running RIP in a complex environment can sometimes cause the flapping of routes. Route flapping refers to the constant deletion and reinsertion of a route within the routing table. To check whether the routes are indeed flapping, check the routing table and look at the age of the routes. If the ages are constantly being reset to 00:00:00, this means that the routes are flapping.

One of the most common reasons is packet loss, which occurs when the packet is dropped on the sender's or receiver's interface. The most common environment this occurs in is Frame Relay, which is used in this scenario.

The existence of packet loss can be verified by utilizing the show interface command and examining the interface statistics to determine if the number of packet drops is constantly incrementing. Figure shows the Frame Relay scenario used in this example.

Debugs and Verification
In a large network, especially in a Frame Relay environment, there is a high possibility that when RIP updates are lost, it is occurring in the Frame Relay cloud or that the RIP interface dropped the update. These symptoms can occur in any Layer 2 media, but this example will involve Frame Relay.

By default, if RIP does not receive a route for 180 seconds, the route is put in a holddown for 240 seconds and then it is purged. This situation is corrected by itself, in time, when a new update is received.

Consider the output in Figure , where no RIP updates have been received for 2 minutes and 8 seconds into the fifth update.

Figure shows that there are a large number of broadcast drops on the interface.

Solution
The show interfaces serial 0 command further proves that there is some problem at the interface level. Too many drops are occurring at the interface. This is the cause of the route flapping. In the case of Frame Relay, the Frame Relay broadcast queue might have to be tuned. Several white papers on Cisco’s web site discuss how to tune the Frame Relay broadcast queue. In a non-Frame Relay situation, the input or output hold queue might need to be increased.

Figure shows that after fixing the interface drop problem, the route flapping disappears.

The output from the show ip route rip command displays that the routes are stable in the routing table and that the timers are set at a value lower than 30 seconds.