OSPF: Neighbor Establishment

The following process occurs when two OSPF neighbors recognize each other and attempt to be come an adjacent fully sync’d neighbor:

OSPFNeighborEstablishment.PNG

OSPF routers go through a series off states when establishing a new neighborship.  Here are the states and their meanings:

Down State- Neighbors start with this state, this is the state that OSPF routers go to when neighborships have been torn down or if no OSPF hellos have been received before the DEAD interval, or when the neighbor is manually configured to the shutdown.

Attempt State – This is only available on NBMA and point to multi-point links.  If a neighbor does not respond to hellos then the neighbor goes back to the down state.

Init state – If a valid Hello packet has been received then the neighbor goes into the Init state.  This means that this router can hear the other router, but does not know if the remote router can hear it.

2-Way state – If a valid Hello packet has been received and the list of seen routers in the Hello includes its own RID, then that adjacency goes into a 2-way state.  This confirms that Hellos are being received bidirectionally.

ExStart state – the neighbor is moved into the ExStart state when it determines that it wants to become fully adjacent with the neighbor.  The purpose of this state is to determine the Master/Slave relationship as well as determine a starting sequence number to acknowledge DBD packets.

Exchange state – A neighbor is moved to exchange when the master/slave have been determined in the relationship.  DBD packets are exchanged and begin exchanging LSAs for the specific networks each device is not aware of in its own DBD.

Loading state – neighbors are moved into loading once the router has advertised its complete list of LSAs and requests to download the LSAs it doesn’t have from the neighbor.  The neighbor remains in the loading state until all LSAs are downloaded.

Full state – A neighbor is put into the full state when all required LSAs have been downloaded and all missing out out dated LSAs have been acquired.  This means that the neighbors are in a stable fully adjacent state.

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