Introduction to OSPF

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For more information on configuring access lists, see the Cisco IOS configuration guides.

Possible Problem: Virtual link and stub area configurations are mismatched.

Solution: Perform the following procedure:

This section might not be applicable to you because this has been fixed in 11.2(11.1) and 11.3(1.1). Virtual links are no longer allowed if area is stub.
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 ABR1 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 
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.

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.

Possible Problem: OSPF routers are not establishing neighbors.

Solution: Follow the procedures outlined in the section “OSPF: Routers Not Establishing Neighbors” earlier in this chapter.

Possible Problem: Routing information from IGRP or RIP is not redistributed correctly into OSPF

Solution: Perform the following procedure:

1.  Check the router configuration by using the show running-config privileged EXEC command.
2.  Look for a redistribute router configuration command entry. Make sure that redistribution is configured and that the subnets keyword is used with the command.
The subnets keyword must be included when IGRP or RIP is redistributed into OSPF; otherwise, only major routes (not subnet routes) are redistributed.
3.  If the redistribute command is not present, or if the subnets keyword is not specified, add or change the configuration by using the following commands:
    GARNER_NC(config)# router ospf 100    GARNER_NC(config)# redistribute igrp subnets    GARNER_NC(config)# redistribute rip subnets 

Possible Problem: There is no ABR configured in an area, which isolates that area from the OSPF backbone.

Solution: Perform the following procedure:

1.  Use the show running-config privileged EXEC command on OSPF routers to verify that at least one ABR exists for the area. ABRs must belong to area 0, the OSPF backbone, as well as to another area. Look for network statements that indicate that the router is part of area 0.
2.  If no ABR exists in an area, configure one where appropriate. Use the network router configuration command.
3.  For example, to configure OSPF process 100 to participate in the OSPF backbone area, enter the following commands:
    C4500(config)# router ospf 100    C4500(config-router)# network area 0 

Possible Problem: There is an interface network type mismatch on Frame Relay WAN.

Solution: In an OSPF Frame Relay environment, if one end of the link is a multipoint interface and the other end is a point-to-point interface, by default the multipoint interface will advertise the link as a non-broadcast network and the point-to-point interface will advertise the link as a point-to-point network. This creates a conflict in the link-state database and can prevent routing information from being learned properly.

1.  Check each router interface on each side of the link to see if the network types are mismatched. Use the show ip ospf interface privileged EXEC command to check the network type configured for the interface. The following is an example of the output from the show ip ospf interface command:
    Ethernet0 is up, line protocol is up    Internet Address, Area 0    Process ID 1, Router ID, Network Type BROADCAST,    Cost: 10    [...] 

In this example, the network type is broadcast.
2.  Change the point-to-point interface to a multipoint interface by configuring subinterfaces, or change the network type of the point-to-point interface to broadcast by using the ip ospf network broadcast interface configuration command.
3.  At this point, your changes will not take affect until you reload the router.

For information on configuring subinterfaces, see the Cisco IOS configuration guides.

Possible Problem: The area is configured as a stub area.

Solution: Route redistribution is not possible in OSPF stub areas. No external routes are advertised into a stub area, and if the area area-id stub no-summary router configuration command is used, no summary routes (inter-area routes) will be advertised into the stub area. Perform the following procedure:

1.  If you want summary routes to be advertised into the stub area, but you do not see them in the routing table, use the show running-config privileged EXEC command to view the router configuration.
2.  Look for an area area-id stub no-summary command entry. If this command is present, disable it by entering the following commands:
    OSPF_STUB_RTR(config)# router ospf 100    OSPF_STUB_RTR(config-router)# no area 1 stub no-summary 

This disables the no-summary keyword and keeps the router configured as a stub.
3.  To advertise external routes into the area, you must configure the area as a non-stub. Make certain that all routers in the area are reconfigured as non-stub routers.

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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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