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.
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.
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.
