When an Open Shortest Path First (OSPF) neighbor router is powered on, it transitions through the following neighbor states:
-Down
-Init
-2-Way
-Exstart
-Exchange
-Loading
-Full

An OSPF neighbor router begins in the Down state. A neighbor in the Down state has not yet sent a hello packet. When a hello packet is received from the neighbor router but the hello packet does not contain the receiving router's ID, the neighbor router is in the Init state. The receiving router replies to the neighbor router with a hello packet that contains the neighbor router's ID as an acknowledgment that the receiving router received the neighbor's hello packet. If a router is stuck in the Init state, it has sent hello packets but has not received them from the neighbor router. If a router is stuck in the Down or Init state, you should check to see whether an access list is blocking 224.0.0.5, which is used by OSPF to send hello packets. Additionally, you should ensure that Layer 1 and Layer 2 connectivity exists and that authentication is disabled or enabled on both neighbors.

The neighbor router replies with a hello packet that contains the receiving router's ID. When this occurs, the neighbor router is in the 2Way state. At the end of the 2Way state, the designated router (DR) and backup designated router (BDR) are elected for broadcast and nonbroadcast multiaccess (NBMA) networks. On broadcast and NBMA networks, neighbor routers will proceed to the Full state only with the DR and BDR; routers will remain in the 2Way state with all other neighbor routers. Routers that remain in the 2Way state will contain 2WAY/DROTHER in the output of the show ip ospf neighbor command. If all routers on a segment remain in the 2Way state, you should verify whether all routers on the segment are set to a priority of 0, which prevents any of them from becoming the DR or BDR.

After the DR and BDR are elected, neighbor routers form master-slave relationships in order to establish the method for exchanging linkstate information. Routers in this state are in the Exstart state. Neighbor routers then exchange database descriptor (DBD) packets. These DBD packets contain linkstate advertisement (LSA) headers that describe the contents of the linkstate database. Routers in this state are in the Exchange state. If a router is stuck in the Exstart or Exchange state, you should determine whether there is a problem with mismatched maximum transmission unit (MTU) settings.

Routers then send linkstate request packets to request the contents of the neighbor router's OSPF database. The neighbor router replies with linkstate update packets that contain the routing database information. Routers in this state are in the Loading state. If a router is stuck in the Loading state, you should determine whether there is a problem with corrupted LSAs.

After the OSPF databases of neighbor routers are fully synchronized, the routers transition to the Full state, which is the normal OSPF router state. A router will periodically send hello packets to its neighbors to indicate that it is still functional. If a router does not receive a hello packet from a neighbor within the dead timer interval, the neighbor router will transition back to the Down state.