The Hello Protocol is responsible for neighbour acquisition, maintenance and for ensuring two way communication between neighbours.
In other words, Hello packets are used to form neighbour/adjacency relationship on OSPF enabled links.
There are 8 STATE process to establish adjacency.
> Initial state of neighbour conversation.
> no Hellos received on OSPF enabled interface in the last RouterDeadInterval (40 Seconds by default – same as dead interval), but can send Hello to the neighbour.
> Only used for manually configured NBMA networks.
> A Router sends unicast Hello packet to OSPF enabled interface, but not received any.
> Hello packet has been seen by the neighbour, But my Router-ID has not been listed in neighbour list.
> This is the first step of adjacency establishment.
> Bi-directional Communication happen between two Routers.
> Hello packet has been seen by the neighbour in last RouterDeadInterval and my Router-ID is now listed in neighbour list.
> BDR and DR is elected in this state.
> If network type is Broadcast or Non-Broadcast Multi-Access (NBMA) then Router becomes FULL, only with DR and BDR, and it stays in 2-WAY state with remaining routers.
> If network type is Point-to-Point or Point-to-Multipoint then Router become FULL with connected Routers.
> Election happens to decide who will be the first, between pair of Routers, to share the database and whose sequence number will follow for future communication.
> Highest Router-ID wins the election and becomes the Master. This election process is on per neighbour basis.
> Sequence number of Master will follow and it acknowledged by Slave.
> Master Router starts exchanging DBD packets and it contains only LSA headers.
> Each DBD packet has sequence number and it is acknowledged by Slave Router.
> Link State Requests (LSR) can also be sent in this state for recent information.
> Link State Updates (LSU) received from neighbour routers which was asked in EXCHANGE state.
> Neighbours are fully adjacent in this state. show ip osfp database command can be used to view all received LSAs.