The OSPF neighbor states are listed below in order of progressing functionality:
Down: This is the initial state of a neighbor conversation. It indicates that there has been no recent information received from the neighbor. On non-broadcast networks, Hello packets may still be sent to "Down" neighbors, although at a reduced frequency.
Attempt: This state is only valid for neighbors attached to non- broadcast networks. It indicates that no recent information has been received from the neighbor, but that a more concerted effort should be made to contact the neighbor. This is done by sending the neighbor Hello packets at intervals of HelloInterval.
Init: In this state, an Hello packet has recently been seen from the neighbor. However, bidirectional communication has not yet been established with the neighbor (i.e., the router itself did not appear in the neighbor's Hello packet). All neighbors in this state (or higher) are listed in the Hello packets sent from the associated interface.
2-Way: In this state, communication between the two routers is bidirectional. This has been assured by the operation of the Hello Protocol. This is the most advanced state short of beginning adjacency establishment. The (Backup) Designated Router is selected from the set of neighbors in state 2-Way or greater.
ExStart: This is the first step in creating an adjacency between the two neighboring routers. The goal of this step is to decide which router is the master, and to decide upon the initial DD sequence number. Neighbor conversations in this state or greater are called adjacencies.
Exchange: In this state the router is describing its entire link state database by sending Database Description packets to the neighbor. Each Database Description Packet has a DD sequence number, and is explicitly acknowledged. Only one Database Description Packet is allowed outstanding at any one time. In this state, Link State Request Packets may also be sent asking for the neighbor's more recent advertisements. All adjacencies in Exchange state or greater are used by the flooding procedure. In fact, these adjacencies are fully capable of transmitting and receiving all types of OSPF routing protocol packets.
Loading: In this state, Link State Request packets are sent to the neighbor asking for the more recent advertisements that have been discovered (but not yet received) in the Exchange state.
Full: In this state, the neighboring routers are fully adjacent. These adjacencies will now appear in router links and network links advertisements.
