OSPF States

OSPF States:

There are 8 states in a ospf adjacency formation.

1. Down
2. Attempt
3. Init
4. 2 way
5. Extart
6. Exchange
7. Loading
8. Full

 1. Down :- After configuring the ospf process, router waits for the hello packet from neighbors. Till the time no hello is received, ospf state is Down.

2. Attempt :- Attempt is the state only in case of NBMA network type. Since neighbor statement is configured manually in case of NBMA network, hence router knows which neighbor it has to form the adjacency with. which is not known in case of other network type.

3. Init :- Once the router receives a hello packet but not the router ID of the neighbor, it sends back a hello packet along with it's router ID. Once both routers get the router ID of each other, routers enter to the 2 way state.

4. 2 Way :- After entering into this state, DR/BDR election is done. Router with highest router ID is elected as DR.

5. Extart :-  After DR/BDR election, master/slave relationship is formed. Master initiates the sequence number of database and it followed by the slave. Any router DR or BDR can be master.

6.Exchange :- Once master slave relationship is formed, DBD (Database descriptor) packets are exchanged. DBD's are actually a snapshot of link states of each routers. Each router compares the link state database of it's own and finds which routes not available in the routing database.

7. Loading :- On the basis of difference found on Exchange state, Link state Request (LSR) and Link State Update (LSU) are exchanged. The which wants the information about any link state, sends a LSU packet to it's neighbor and neighbor replied with database information which is known as LSU.

8. Full :- Once the database synchronization is completed between neighbors, adjacency is formed and state is Full.