BGP Neighbor-ship States Explained

1. Idle – Idle is normally referred to be as Down, or no route to the neighbor,
or if something fails in to form a BGP neighbor-ship(eg: TCP 3 way handshake), it again moves back to Idle State for short period of time.
2. Connect – Receiving a Packet from Neighbor (Neighbor will be in Active State ).
It detects the incoming packet.
3. Active – Initiation of packet to form Neighbor-ship, or it re-tries to Establish a session again, and it waits for 3 way handshake to complete. BGP is to be in couple of seconds in Active state, or it seems something to be wrong in BGP.
4. Open-Sent – Once TCP-3-way handshake is completed, Open-Sent message is sent to the neighbor.
In this state it will sent a BGP Open message to the neighbor.
5. Open-Confirm – Once the Neighbor receives the Open-Sent message, it will look into some parameters (AS Number, Prefix , etc ,…) , if every thing is OK, it will sent a Open message back to the neighbor. If anything seems to be not OK, it kills the TCP session and will send a Notification message to the neighbor. It will not sent a Open-Confirm message.
6. Established – Once the Router send and receives the Open message, it will move to Established State, and start Exchanging Updates.

 

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