As shown in the figure, OSPF problems usually relate to:
- Neighbor adjacencies
- Missing routes
- Path selection
When troubleshooting neighbor issues, verify if the router has established adjacencies with neighboring routers using the show ip ospf neighbors command. If there is no adjacency, then the routers cannot exchange routes. Verify if interfaces are operational and enabled for OSPF using the show ip interface brief and the show ip ospf interface commands. If the interfaces are operational and enabled for OSPF, ensure that interfaces on both routers are configured for the same OSPF area and the interfaces are not configured as passive interfaces.
If adjacency between two routers is established, verify that there are OSPF routes in the routing table using the show ip route ospf command. If there are no OSPF routes, verify that there are no other routing protocols with lower administrative distances running in the network. Verify if all of the required networks are advertised into OSPF. Also verify if an access list is configured on a router that would filter either incoming or outgoing routing updates.
If all of the required routes are in the routing table, but the path that traffic takes is not correct, verify the OSPF cost on interfaces on the path. Also be careful in cases where the interfaces are faster than 100 Mb/s, because all interfaces above this bandwidth have the same OSPF cost, by default.