System information
Troubleshooting TCP/IP 7-107
TCP/IP: Routes Learned from the Wrong Interface or Protocol
TCP/IP: Routes Learned from the Wrong Interface or Protocol
Symptom: Routes in the routing table were learned from the wrong interface or protocol. For
example, networks that should be reached through one interface are shown in the routing table to be
reachable through another interface instead. This problem occurs only in a multiprotocol
environment (see the section “Split Horizon Example,” later in this chapter).
Table 7-4 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 7-4 TCP/IP: Routes Learned from the Wrong Interface or Protocol
Split Horizon Example
Sometimes in a multipoint WAN environmentit is desirable to leave split horizon disabled. However,
steps should be taken to prevent routing information from being learned from the wrong interface or
protocol. For example, in the environment shown in Figure 7-11, Router 2 might incorrectly receive
information about RIP networks from Router 3 if the routers are not configured correctly.
Figure 7-11 A Split Horizon Sample Network
Possible Problem Solution
Split horizon is disabled Step 1 Use the show ip interface privileged exec command on the remote router
to see the router configuration.
Step 2 Make sure that split horizon is enabled. Check the output of the show ip
interface command for the following:
Split horizon is enabled
Step 3
If split horizon is not enabled, enter the ip split-horizon interface
configuration command on the remote router interface.
For example, to enable split horizon on serial interface 0, enter the
following commands:
C4500(config)#interface s0
C4500(config-if)#ip split-horizon
Note: The default split-horizon setting for all LAN interfaces is enabled.
However, for WAN multipoint interfaces configured with X.25, Frame Relay, or
SMDS
1
encapsulation, the default split-horizon setting is disabled.
1 SMDS = Switched Multimegabit Data Service
WAN
S0
S0
Network
193.10.1.0
255.255.255.0
E1
IGRP RIP
Network
169.170.69.0
255.255.255.0
Router 3
Router 2
Router 1