System information

Troubleshooting TCP/IP
Book Title
7-128
Possible Problem Solution
Misconfigured or
missing network
router configuration
command
Step 1 Use the show ip eigrp neighbors exec command on an Enhanced IGRP
router. Make sure that all directly connected Enhanced IGRP routers
appear in the output.
Step 2 If some connected routers are not shown in the output, use the show
running-config privileged exec command to view the configuration of
the suspect routers.
Step 3 Make sure that a network router configuration command is specified for
every network to which a router interface belongs.
For example, if the IP address of Ethernet interface 0 is 192.168.52.42,
enter the following commands to enable Enhanced IGRP on the interface:
c4500(config)#router eigrp 100
c4500(config-router)#network 192.168.52.0
Mismatched
autonomous
system number
specification
Step 1 View the router configuration using the show running-config privileged
exec command on each router in the autonomous system.
Step 2 Check the router eigrp global configuration commands to make sure that
all routers that should be communicating are in the same autonomous
system.
Only Enhanced IGRP routers in the same autonomous system will form neighbor
relationships and thus exchange routing information.
Access list is
misconfigured
Step 1 Execute the debug ip packet and debug eigrp packets privileged exec
commands. The former command indicates whether IP packets are being
sent and received and whether there are encapsulation problems. The
latter command indicates whether Enhanced IGRP hello packets are
being sent and received properly.
Caution: These debug commands can use considerable CPU cycles on
the router. Do not enable them if your network is already heavily
congested.
Step 2 If a router appears to be sending IP and Enhanced IGRP packets correctly,
but a connected router does not receive them, check the configuration of
the connected router for access lists that might be filtering out packets.
Step 3 Disable all access lists enabled on the router using the no ip
access-group access-list-number in interface configuration command.
Step 4 Monitor the output from the debug ip packet and debug eigrp packets
commands. Determine whether packets are now being received normally.
Step 5 If packets are received normally, an access list is probably filtering
packets. To isolate the problem list, enable access lists one at a time until
packets are no longer forwarded.
Step 6 Check the access list to see whether it is filtering traffic from the source
router. If it is, alter the access list to allow the traffic to pass. Enter explicit
permit statements for traffic that you want the router to forward normally.
Step 7 Enable the altered access list with the ip access-group command to see
whether packets continue to pass normally.
Step 8 If packets pass normally, perform the preceding steps on any other routers
in the path until all access lists are enabled and packets are forwarded
properly.