Before exchanging routing information, routers running Open Shortest Path First (OSPF) form neighbor relationship with other OSPF routers on the same segment by exchanging hello packets. The hello packets contain various parameters. Some of them should match between neighboring routers. These include:
Hello and Dead intervals
Authentication type and password
Stub Area flag
Subnet ID and Subnet mask
If any of these parameters do not match, the router from which the hello packets are received is not considered as a neighbor. Adjacency establishment does not proceed further, resulting in routing failures.
The mismatch occurs primarily due to configuration errors related to any of these parameters. The hello and dead interval mismatches are more common when OSPF operates over Non-Broadcast Multi-Access (NBMA) networks such as Frame Relay and ATM hub and spoke environments, where physical interfaces are used on one end and point-to-point sub-interfaces are used on the other end. This results in different OSPF network types on either ends, using different hello and dead interval timers when they are left with the default configurations.
Issue the show ip ospf neighbor command from the privileged EXEC mode to identify the adjacency state with the neighbor. When a mismatch exists, the output of this command is empty if the neighbors are trying to be discovered automatically. If the neighbors are statically configured, the neighbor is displayed in DOWN state.
If neighbor relationships are not being formed over an interface, issue the show ip ospf interface command to view the hello and dead intervals, IP address, subnet mask and Area ID parameters. You can use the show ip ospf command to identify if any stub areas are configured.
You can also use the debug ip ospf hello and debug ip ospf adj commands from the privileged EXEC mode to identify which parameter has a mismatch.
To modify area to which the interface belongs, use the router ospfprocess-id command in the global configuration mode, followed by the no networkip-addresswildcard-mask area area-id command under the router configuration mode to remove the interface from the specific area, and then issue the network ip-address wildcard-mask area area-id command to include the interface under the correct area.
To modify the authentication type, issue the areaarea-idauthentication[message-digest] command under the router configuration mode or the ip ospf authentication[message-digest | null] command under the interface configuration mode. To configure the correct passwords, use the ip ospf authentication-keypassword or ip ospf message-digest-keykey-idmd5key command under the interface configuration mode.
To set the stub area flag, configure the area as stub or Not-So-Stubby-Area (NSSA), by issuing the areaarea-idstub or areaarea-idnssa commands from the router configuration mode.
To change the IP address and subnet mask, issue the ip addressip-addressmask command under the interface configuration mode.