OSPF Neighbor Adjacency States

Before you start

Objectives : You will be able to understand how OSPF router become neighbor of each other.

Prerequisites: You have to know about OSPF protocol.

Tags : OSPF , Routing Protocol, Neighborship States.

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Introduction to OSPF Adjacency

I have seen that people have some problem to understand that how OSPF router become neighbors each others. So, in this article i am going to show you, which states a OSPF network router must follow and then become neighbor of each other.

OSPF Neighbor Adjacency States:- Down, Attempt, Init State, Two-way, Ex-start, Ex-load, Ex-change, Full load.

1. Down : In Down state, OSPF Hello message are send from one side. Router at another end not receive these Hello Message, because currently OSPF is not enabled on that router.

2. Attempt : Attempt state is also like to Down state, but this state is valid for only Non-Broadcast Multiaccess (NBMA) network. So, NBMA network must for this state. Also, neighborship is statically defined in this state.

3. Init State : In this state both router receive hello from each other.

4. Two-way state : In this state, Router send a Hello packet along with Neighbor-ID (i.e. Neighbor router ID) & other router receive it, then they are in 2-way state.

Note: If our links is Multiaccess (i.e Ethernet), then DR/BDR election are done in this state.

5. Ex-start : We know, In OSPF, every router create a database, and thus all router have a knowledge of complete topology. This Database can be formed with the help of various LSA's.
In this state, router will exchange empty, DBD (Database Description), for Master and Slave election.
Router will negotiate the sequence number of DBD & MTU size.

Master & Slave Election: Router, which have highest router-ID will become Master, and other will be slave. Here, Master router first send the LSA's headers to slave router in next state (i.e Ex-change).

6. Ex-change : Here, Master router initiate sending the LSA's Header to slave router with sequence number.

Note: These are just LSA's header, not detailed information.

7. Loading : In Ex-change state, both router share only these LSA header, In this state, both router get detailed information of pending LSA's using LSR (Link State Request), LSU (Link State Update), LSAck (Link state Acknowledgement).

8. Full :

It is the last state of OSPF neighborship. In this state, router have synchronized database successfully.