Cisco "debug ip ospf adj" Command Explained
debug ip ospf adj — streams live OSPF adjacency-formation messages to the console — showing exactly where two routers get stuck when they fail to become neighbors. Runs in privileged EXEC mode.
Syntax and common variants
| Variant | Purpose |
|---|---|
debug ip ospf adj | Watch adjacency state changes in real time |
undebug all (or u all) | Stop all active debugs immediately |
Reading the output
| Output / element | Meaning |
|---|---|
Nbr … 2WAY -> EXSTART | Progressing normally through states |
Nbr … stuck in EXSTART | Election or MTU issue — investigate immediately |
Mismatch area ID / hello timers | The debug prints the specific mismatch causing failure |
When to use it
Reserved for active troubleshooting, not routine use — debug output can be heavy on a busy router. When OSPF neighbors won't form and show commands alone don't reveal why, this shows the actual negotiation messages and exactly which parameter mismatches. Always run undebug all when done. Browse more in the command reference or practise in the free labs.
Frequently asked questions
Is debug ip ospf adj safe to run on a production router?
Use cautiously — debug commands add CPU load. On a busy router, consider conditional debugging or a maintenance window; always undebug all when finished.
What does 'stuck in EXSTART' in the debug output usually mean?
Almost always an MTU mismatch between the two routers' interfaces — the exchange of database description packets fails to progress.
How do I stop a running debug?
undebug all (or its shortcut u all) immediately disables all active debug output.
Related articles
Want hands-on training?
Learn this on real Cisco lab devices with placement support at Attila Technologies, Ahmedabad.