System information

Troubleshooting TCP/IP
Book Title
7-126
Misconfigured
route filtering
Step 1 Use the show running-config command to check suspect routers.
Step 2 See if there are any distribute-list in or distribute-list out router
configuration commands configured on the router.
The distribute-list in command prevents specific information learned in
LSAs
1
from being included in the OSPF routing table. The distribute-list
out command prevents a router from including specific information in
routing updates that it transmits. However, in OSPF, distribute-list out
can be configured only on an ASBR
2
to filter external routes.
Note: Although distribute-list commands prevent specific information
from being included in the OSPF routing table, information about those
networks is contained in the link-state database and is flooded through the
network in LSAs. This means that downstream routers will include that
information in their routing tables unless they, too, filter those routes from
the routing table.
Misconfigured
route filtering
Step 3 If distribute-list commands are configured on the router, disable them
using the no version of the command.
For example, to disable an incoming filter that references access list 10,
enter the following command:
C7500(config)#no distribute-list 10 in
Step 4
After disabling all distribution lists, use the clear ip route privileged exec
command to clear the routing table.
Step 5 Determine whether the routes appear in the routing table by using the
show ip route privileged exec command. If routes appear properly in the
routing table, the access list referenced by the distribute-list command is
probably configured to deny certain updates.
Step 6 To isolate the problem list, enable distribution lists one at a time until the
routes no longer appear in the table.
Step 7 Use the show running-config command and check the access list to make
sure it does not deny updates inappropriately. If the access list denies
updates from specific addresses, make sure that it does not deny the
address of a router from which routing updates should be received.
Change the access list to allow the router to receive updates from the
proper addresses. 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 is blocked. Configure explicit permit
statements for those addresses from which the router should receive
updates.
Step 8 If you altered an access list, enable the distribution list using the
distribute-list command. Use the clear ip route command and check
whether the missing routing information appears in the routing table.
Step 9 If the routes appear in the routing table, perform the preceding steps on
every router in the path until all distribution lists are enabled and routing
information appears properly in the routing table.
For more information on configuring access lists, see the Cisco IOS configuration
guides.
Possible Problem Solution