OSPF Neighbor States
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Two routers aren't exchanging routes just because they're cabled together — they walk a strict state machine from silence to a fully synchronized LSDB.
| State | What's happening |
|---|---|
| Down | Nothing heard from the neighbor |
| Init | Their hello seen — but they haven't seen ours yet |
| 2-Way | Both see each other; DROTHER pairs stop here by design |
| ExStart | Master/slave negotiation for database exchange |
| Exchange | Database Description (DBD) summaries are swapped |
| Loading | Missing LSAs requested via LSR, delivered via LSU |
| Full | Identical LSDBs — actual routing can happen |
Debugging quick hits
- Stuck in Init — one-way visibility: ACL or authentication dropping your hellos.
- Stuck in 2-Way — normal between two DROTHERs on a LAN; only pairs involving the DR/BDR go Full.
- Stuck in ExStart/Exchange — the classic MTU mismatch.
- Flapping Full ↔ Down — hello/dead timer mismatch or an unstable link.
Hellos must agree on…
Area ID, hello/dead timers, authentication, stub flags, and subnet — any mismatch and the adjacency never leaves Init. Router IDs must differ; duplicate RIDs are their own special outage.