What are the OSPF neighbour states?
Before establishing a neighbor relationship, OSPF routers need to go through several state changes. These states are explained below.
Init state: A router has received a Hello message from the other OSFP router
2-way state: The neighbour has received the Hello message and replied with a Hello message of his own
Exstart state: It is beginning of the LSDB exchange between both routers. Routers are starting to exchange link state information.
Exchange state: DBD packets are exchanged. DBDs contain LSAs headers. Routers will use this information to see what LSAs need to be exchanged.
Loading state: one neighbor sends LSRs (Link State Requests) for every network it doesn’t know about. The other neighbor replies with the LSUs (Link State Updates) which contain information about requested networks. After all the requested information have been received, other neighbor goes through the same process
Full state: The Both routers have the synchronized database and are fully adjacent with each other.