OSPF neighbor states: from Hello to FULL
Down, Init, 2-Way, ExStart, Exchange, Loading and Full: follow one adjacency without panicking at every intermediate state.
The first conversation
Down means no recent Hello has been received. In Init, a Hello has arrived but your own router ID is not yet visible in the neighbor’s Hello. In 2-Way, each side has seen itself in the other’s neighbor list.
Should these routers become adjacent?
Not every neighbor pair needs a full adjacency. On a broadcast network, two DROther routers normally stay 2-Way with each other. Each forms full adjacencies with the DR and BDR. A point-to-point link follows a different adjacency model.
Synchronising the database
ExStart negotiates database-description exchange roles and sequence numbers. Exchange sends database-description packets that summarise LSDB contents. Loading requests and receives missing or newer LSAs. Full means the adjacency’s databases have synchronised.
A less common state
Attempt is used for configured neighbors on NBMA networks. It is not a normal stage on every Ethernet adjacency.
A state is a clue
Persistent Init suggests one-way communication. ExStart or Exchange can point to an MTU mismatch, but also packet loss or negotiation problems. Loading suggests requested LSAs are not arriving or being accepted. Always inspect evidence before changing configuration.
First checks
On Cisco IOS-style CLIs, start with show ip ospf neighbor and show ip ospf interface. Confirm the interface, area, timers, network type and authentication. Commands and defaults vary by platform.
Download the neighbor-state sheet.
Protocol reference: RFC 2328 (OSPFv2). NSSA reference: RFC 3101.