OSPF Verification and Troubleshooting

You've configured OSPF on both routers. `show ip ospf neighbor` on R1 shows no neighbors. `show ip ospf neighbor` on R2 shows R1 in INIT state. Both routers have OSPF enabled on the connecting interface with matching area IDs. Where do you look next?

INIT state is one-sided — R2 receives R1's Hellos but R1 doesn't receive R2's. That's a one-way communication problem. Nine times out of ten it's a firewall or ACL blocking UDP 224.0.0.5 (OSPF multicast) in one direction. Ten times out of ten, you can confirm it in five seconds.

#OSPF Failure Categories#link

#Diagnosing with show Commands#link

The key lines are `State is FULL`, `Options is 0x52 in Hello`, and the Dead timer. If State isn't FULL and the neighbor is important, the state tells you which phase failed — INIT (one-way issue), ExStart (MTU), Exchange (LSA exchange issue), Loading (LSDB issues).

#Using Debug Carefully#link

#Structured OSPF Troubleshooting#link

#Check Your Understanding#link

#Common Mistakes#link

• ▪Running `debug all` — can CPU-lock the router in seconds
• ▪Forgetting `undebug all` after debugging — debug output continues until disabled
• ▪Assuming 2-WAY is broken — it's normal on broadcast for non-DR/BDR
• ▪Ignoring `show logging` — OSPF logs the reason for neighbor failures clearly
• ▪Not checking MTU — it's the hidden cause of ExStart stalls