OSPF : Working states

 OSPF :-

Open Shortest Path First (OSPF) is a link-state routing protocol that is used to find the best path between the source and the destination router using its own Shortest Path First). It is a network layer protocol which works on the protocol number 89 and uses AD value 110. OSPF uses multicast address 224.0.0.5 for normal communication and 224.0.0.6 for update to designated router(DR)/Backup Designated Router (BDR).

OSPF states – The device operating OSPF goes through certain states. These states are:

1.       Down – In this state, no hello packet have been received on the interface.
Note – The Down state doesn’t mean that the interface is physically down. Here, it means that OSPF adjacency process has not started yet.

2.       INIT – In this state, hello packet have been received from the other router.

3.       2WAY – In the 2WAY state, both the routers have received the hello packets from other routers.   Bidirectional connectivity has been established.

        Note – In between the 2WAY state and Exstart state, the DR and BDR election takes place.

4.       Exstart – In this state, NULL DBD are exchanged.In this state, master and slave election take place. The router having the higher router I’d becomes the master while other becomes the slave. This election decides Which router will send it’s DBD first (routers who have formed neighbourship will take part in this election).

5.       Exchange – In this state, the actual DBDs are exchanged.

6.       Loading – In this sate, LSR, LSU and LSA (Link State Acknowledgement) are exchanged.
Important – When a router receives DBD from other router, it compares it’s own DBD with the other router DBD. If the received DBD is more updated than its own DBD then the router will send LSR to the other router stating what links are needed. The other router replies with the LSU containing the updates that are needed. In return to this, the router replies with the Link State Acknowledgement.

7.       Full – In this state, synchronization of all the information takes place. OSPF routing can begin only after the Full state.