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Chapter 6

Troubleshooting TCP/IP Connectivity

debug Commands

The debug

command should always be used with caution and, in many cir-

cumstances, as a last resort. If the previous show commands do not provide
you with enough information to isolate and resolve the RIP problem, you
can enable the debug tool.

The syntax for the debug mode in RIP is debug ip rip events. If you

need even more general RIP information, use the global form of the com-
mand, debug ip rip. This command provides you with all possible RIP pro-
tocol information.

Typical Problems

Because RIP uses UDP broadcasts by default, it can cause network conges-
tion or broadcast storms if not configured correctly. The way to avoid this
problem is to configure RIP to allow unicast updates. This is done with the
neighbor

statement from within the RIP protocol configuration mode. In

addition to using the neighbor statement, specified interfaces can be made
passive-interfaces

. This means that routing updates are not sent out to

the specified interfaces. Frequent routing updates can also cause network
congestion. This can be controlled or remedied by adjusting the various RIP
timers.

Problems can also occur due to version mismatches. By default, Cisco

routers can understand both versions, but they advertise and forward data
using RIP1. It is possible to configure interfaces to send and receive only one
version. The problem occurs when the RIP versions on the two connected
interfaces do not match.

For example, if Router A's interfaces are configured to send and receive

only RIP2, and Router B's interfaces are configured to listen and speak RIP1,
the two routers won't be able to share RIP information. This problem can be
resolved by analyzing the interface configuration on both routers and chang-
ing them so they match.

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