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Route is missing from the routing table due to OSPF forwarding address check

Core Issue

The Open Shortest Path First (OSPF) forwarding address concept allows another router's IP address to be specified as the forwarding address to avoid an extra hop in the path to the destination. This is typically seen when the next-hop router for a network is connected to the same multi-access segment. The forwarding address is accepted by an OSPF router (and the corresponding route installed in the routing table) if the route to the forwarding address is learned as an inter-area or intra-area route. However, in certain setups this might not be the case, resulting in the route being dropped.

Resolution

If a route is dropped unexpectedly, issue the show ip route command, along with the specific prefix in the user EXEC or privileged EXEC mode. This checks if the forwarding address is learned as an inter-area or intra-area route. 

If a route is not dropped unexpectedly, certain configuration changes may become necessary. These configuration changes consist of disabling summarization at certain points in the network or preventing redistribution of certain routes.

For more information, refer to Common Routing Problem with OSPF Forwarding Address


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