Explanation:
When OSPF adjacency is formed, a router goes through several state changes before it becomes
fully adjacent with its neighbor. The states are Down -> Attempt (optional) -> Init -> 2-Way ->
Exstart -> Exchange -> Loading -> Full. Short descriptions about these states are listed below:
Down: no information (hellos) has been received from this neighbor.
Attempt: only valid for manually configured neighbors in an NBMA environment. In Attempt state,
the router sends unicast hello packets every poll interval to the neighbor, from which hellos have
not been received within the dead interval.
Init: specifies that the router has received a hello packet from its neighbor, but the receiving
router's ID was not included in the hello packet
2-Way: indicates bi-directional communication has been established between two routers.
Exstart: Once the DR and BDR are elected, the actual process of exchanging link state
information can start between the routers and their DR and BDR.
Exchange: OSPF routers exchange and compare database descriptor (DBD) packets
Loading: In this state, the actual exchange of link state information occurs. Outdated or missing
entries are also requested to be resent.
Full: routers are fully adjacent with each other
(Reference:
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f0e.shtml)