OSPF Neighbor Relationship Process

OSPF neighbor Relationship Process

Routers on the same network segment are called neighbors. Two routers connected with each other become neighbors if they have the same area-id, subnet, authentication and hello/dead intervals. This process involves the following steps and states:

1.     First router determined OSPF Router-id (RID)

2.     Network command add OSPF interfaces into OSPF process and router run link-state   

3.     Hello packets are send out on OSPF running interfaces.

When OSPF router sends a hello message it is in down-state.

Down State:

When router sends a hello packet it is down-state, hello message is send in 10s in broadcast / point to point network and after every 30s in NBMA (non broadcast multi-access). In this state the router has not received any hello packet from the Neighbor.

Attempt State:

This state is only valid when the neighbor ship is manually configured in NBMA (Non-broadcast multi-access) Network. In this state the router sends unicast hellos every poll interval to the neighbor from which hellos have not been received within the dead interval.

INIT State:

Received Hello from other routers, in this state router check hello timer, subnet, area ID and authentication.

Two-way State:

This state indicates that the bi-directional communication has been established between two routers. If already neighbor then update dead timer, if no then add as a new neighbor and move to next step.

Exstart State:

Master slave are selected in this state. A router having high priority will become Master if they have same then higher RID break the tie. Master sends the DBD ( Data base description) first to slave.

Exchange State:

Routers will describe their entire link-state database by sending database description packets. DBD are exchanges in this state.
Loading State:

In loading state router start saving the information in memory. Slave send LSR (link state request), master reply with LSU (link state update), similarly master request for missing info by LSR (link state request) and slave rely with LSU (link state update).

Full State:

In full state neighbors are synchronized, SPF algorithm is run for calculation shortest path.