System information

Troubleshooting TCP/IP 7-123
OSPF: Routes Missing from Routing Table
OSPF: Routes Missing from Routing Table
Symptom: OSPF routes and networks are not being advertised to other routers. Routers in one area
are not receiving routing information for other areas. Some hosts cannot communicate with hosts in
other areas, and routing table information is incomplete.
Access list is misconfigured Step 1 Use the show access-list privileged exec command on suspect routers to
see whether there are IP access lists configured on the router.
Step 2 If there are IP access lists enabled on the router, disable them using the
appropriate commands. For example, to disable input access list 10, use
the following command:
C4000(config-if)#no ip access-group 10 in
Step 3
After disabling all access lists on the router, determine whether the router
is able to establish neighbor relationships normally. Use the
show ip ospf neighbor privileged exec command. If the proper neighbor
relationships have been established, an access list is probably filtering
OSPF hello packets.
Step 4 To isolate the problem access list, enable access lists one at a time until
the router cannot establish neighbors (use the clear ip ospf neighbors
privileged exec command to force the router to clear the neighbor table).
Step 5 Check the access list to see whether it is filtering traffic from port 89, the
port used by OSPF. At the end of every access list is an implied “deny all
traffic” criteria statement. Therefore, if a packet does not match any of
your criteria statements, the packet will be blocked. If an access list
denies OSPF traffic, enter an explicit permit statement for port 89 to
ensure that neighbor relationships can be established properly. (You can
also use the ospf keyword when configuring the access list.)
For example, to configure input access list 101 to allow OSPF traffic to
pass, enter the following on the router:
C4500(config)#access-list 101 permit ospf any any
Step 6
If you altered an access list, enable the list and enter the clear ip ospf
neighbors privileged exec command. Then enter the
show ip ospf neighbor command to see whether neighbor relationships
are established normally.
Step 7 If the router is establishing neighbors, perform the preceding steps for
other routers in the path until all access lists are enabled and the router
can still establish neighbors normally.
For more information on configuring access lists, see the Cisco IOS configuration
guides.
Virtual link and stub area
configuration mismatch
Step 1 A virtual link cannot be configured across a stub area. Check router
configurations for routers configured both as part of a stub area and as an
ABR
2
that is part of a virtual link. Use the show running-config
privileged exec command and look for command entries that are similar
to the following:
area 2 stub
area 2 virtual-link 192.169.100.10
Step 2
If both of these commands are present, there is a misconfiguration.
Remove one of the commands (using the no form of the command) to
resolve the misconfiguration.
1 OSPF = Open Shortest Path First
2 ABR = area border router
Possible Problem Solution