Case Study: Troubleshooting Neighbor Problems
When you execute a show ip ospf neighbor command and it reveals nothing or it shows nothing about the particular neighbor you are analyzing, it indicates that this router has seen no valid OSPF Hellos from that neighbor. Check the following items:
Is the local router or neighboring router’s interface up, with line protocol up? Use the show interface command to find out.
Check for IP connectivity between the neighboring routers as follows:
Can you ping the neighbor?
Does the neighbor respond if you ping 220.127.116.11? (This is the address to which OSPF Hellos are sent.)
Check for any inbound access lists or other devices (such as a switch) that might prohibit sending/receipt of IP packets from one neighbor ...