OSPF Routing Table

A network may be a part of the OSPF database without being installed on the routing table. Some discrepancy is found in the database by the OSPF router that restricts it from installing the network in the routing table. The most common cause is that the router advertising the LSA is not reachable via OSPF because of misconfigurations.

Consider the example depicted in Figure 9.4. The figure shows an OSPF network in which routers A1 and A2, which are connected over a serial link, are a part of the shared LAN.

Figure 9.4: An OSPF network.

Consider in Figure 9.4 the route 10.20.1.0/24 is not installed in the routing tables of A2 and A3.

The IP address configured in the serial link between A1 and A2 is not a part of the same subnet. Adjacency is established between the two routers successfully in a point-to-point link. OSPF does not check if the neighbors belong to the same IP subnet. Even though the neighbor relationship is developed and network 10.20.1.0/24 reaches the OSPF database, it is not installed in the routing table due to the nonreachability of next-hop IP address. This problem is resolved by correctly configuring the IP address at both ends of a serial link. This problem can also occur if one of the interfaces is configured as unnumbered and the IP address is configured on the other interface.

In Figure 9.4, consider that route 10.10.1.2/24 is not established in the routing table of A3. In this case, check if any filter is applied at the incoming direction by using the show ip protocols command. We need to suitably modify the access lists to allow installation of the network in the routing table.