Introduction to OSPF

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Possible Problem: A misconfigured route is filtering.

Solution: Perform the following procedure:

1.  Use the show running-config command to check suspect routers.
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 LSAs1 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 ASBR2 to filter external routes.
Note that 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.
3.  If distribute-list commands are configured on the router, disable them by 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 
4.  After disabling all distribution lists, use the clear ip route privileged EXEC command to clear the routing table.
5.  Determine if 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.
6.  To isolate the problem list, enable distribution lists one at a time until the routes no longer appear in the table.
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. Remember that an implicit deny any ends every access list.
Configure explicit permit statements for those addresses from which the router should receive updates.
8.  If you altered an access list, enable the distribution list by using the distribute-list command. Use the clear ip route command and check to see if the missing routing information appears in the routing table.
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: The virtual link is misconfigured.

Solution: Perform the following procedure:

1.  Check the configuration of the routers at each end of the virtual link by using the show running-config privileged EXEC command.
Look for area area-id virtual-link router-id router configuration command entries. These commands are used to configure the virtual link.
2.  Use the show ip ospf EXEC command to find the router ID (IP address) of the routers.
3.  Add the area area-id virtual-link router-id command if it is missing, or modify it if it is incorrect. Make sure that the proper area ID and router ID (IP address) are specified. The routers at each end of the virtual link must point to one another across the transit area.
For example, a virtual link from Router B to Router A is created across the transit area, Area 1.
The following commands are entered on Router A:
    ROUTER_A(config)# router ospf 250    ROUTER_A(config-router)# network 121.10.0.0 0.0.255.255 area 0    ROUTER_A(config-router)# network 169.192.56.0 0.0.0.255 area 0    ROUTER_A(config-router)# area 1 virtual-link 121.10.100.46 

On Router B, the following commands are used:
    ROUTER_B(config)# router ospf 250    ROUTER_B(config-router)# network 121.10.0.0 0.0.255.255 area 0    ROUTER_B(config-router)# network 108.31.0.0 0.0.255.255 area 2    ROUTER_B(config-router)# area 1 virtual-link 121.10.1.1 


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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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