This state 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. When a router receives a hello packet from a neighbor, it should list the sender's router ID in its hello packet as an acknowledgment that it received a valid hello packet.
2-Way
Reference:
+ Each router compares the DBD packets that were received from the other router: Exchange
+ Routers exchange information with other routers in the multiaccess network: Exstart
+ The neighboring router requests the other routers to send missing entries: Loading
+ The network has already elected a DR and a backup BDR: 2-way
+ The OSPF router ID of the receiving router was not contained in the hello message: Init
+ No hellos have been received from a neighbor router: Down
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 Outdated or missing entries are also requested to be resent.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)