OSPF States

Fig 1: OSPF States

OSPF has to get through 7 states in order to become neighbor these seven states are the following:

1. Down No OSPF neighbors detected at this moment.
2. Init Hello packet received.
3. Two-way Own router ID found in received hello packets
4. Exstart Master and slave roles determined.
5. Exchange Database description packets are sent.
6. Loading Exchange of Link state request and link states updates.
7. Full OSPF Routers now have an adjacency.

Down State

As soon as configure OSPF on the router it will start sending the hello packets and that has no clue about the other OSPF routers at this situation it is in the down state. And the hello packets are will be sent to the
multicast address 224.0.0.5.

Init State

In this state hello packet received and once you configure the OSPF your router will start sending hello packets and if you also received hello packets from the other router you will become the neighbors. There are couples of fields in the hello packet and many of them have to match otherwise you won’t become neighbors.

Two Way State

If the one router send hello packet and other router has to respond to first router with a hello packet. And this packet is not sent using multicast but with unicast and in the neighbor field it will include all OSPF neighbors that the second router has and the first router will see the it’s own name in the neighbor field in this hello packet and first router will receive this hello packet and sees it own router-ID and they both are in the two way state.

Exstart State

At this step we have to select a master and slave role and the router with the highest router-ID will become the master and the other will become the slave.

Exchange State

In the exchange state our routers are sending a DBD with a summary of the link state database. This way the routers can find out what networks they don’t know about.

Loading State

When our routers receive the DBD from the other side they will do a couples of things:

• Send acknowledgment using the Lsack packet.
• Compare the information in the DBD with the information it already has.
• If the neighbor has new or any triggered information it will send a LSR (link State Request) packet to request for this information.
• When the router starts sending a LSR than state is loading state.
• The other router will respond with the Link state update. 

Full State

After the loading state the last state is the full state in which routers have synchronized Link state database and routers are ready to route.