Reference: Routing TCP/IP 2nd Edition Jeff Doyle, page 353-357.
OSPF router goes through several states before adjacency is formed.
DOWN – No hello packets was seen from the neighbour router since the last dead interval. If an OSPF router goes to DOWN state from its higher states, its link state retransmission, database summary and link state request lists are cleared.
ATTEMPT – This state applies to routers in NBMA network, where the neighbours are manually configured. A router sends hello packet to its neighbour at hello interval instead of poll interval.
INIT – Hello packet has been seen from neighbour router in the last dead interval, but there is no two way communication established.
2WAY – This state means the router has seen its own router id from its neighbour’s hello packet. Two way communication is established; in NBMA or broadcast network a router must be in 2way state or higher to be eligible for DR and BDR election. A router that receives the database description packet (DDP) from its neighbour in INIT state will also transit itself to 2WAY state.
EXSTART – The router and its neighbours will establish master/slave relationship and determine the database description sequence number for exchange of database description packets. The router with the highest router id becomes the master.
EXCHANGE – The router sends DDP that describes its entire link state database to neighbours that are in exchange state. The router may also send link state request packets to neighbours to request more recent LSA.
LOADING – The router sends link state request packets to neighbours that are in the loading state, requesting more recent LSAs that have been discovered in the exchange state but have not yet been received.
FULL – The routers are fully adjacent. The adjacencies appear in router LSA and network LSA.
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