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The BGP neighbor states

BGP forms a unique, unicast-based connection to each of its BGP-speaking peers. To increase the reliability of the peer connection, BGP uses TCP (port 179) as its underlying delivery mechanism. The update mechanisms of BGP are also somewhat simplified by allowing the TCP layer to handle such duties as acknowledgment, retransmission, and sequencing. Because BGP rides on TCP, a separate point-to-point connection to each peer must be established.

BGP nghbr states

1. IDLE  :  veryfing route to neighbor
BGP refuses all incoming connections. No BGP resources are allocated in Idle state, and no incoming BGP connections are allowed.
2. Connect  : 
BGP waits for a TCP connection to be completed. If successful, the BGP state machine moves into OpenSent state after sending the OPEN message to the peer. Failure in this state could result in either going into Active state or Connect state, or reverting back to Idle state, depending on the failure reasons.
3. Active  : attempting connectivity to neighbor
In this state, a TCP connection is initiated to establish a BGP peer relationship. If successful, BGP sends its OPEN message to the peer and moves to OpenSent state. Failure can result in going to the Active or Idle states.
4. OpentSent  : open message sent to neighbor
After sending an OPEN message to the peer, BGP waits in this state for the OPEN reply.
If a successful reply comes in, the BGP state moves to OpenConfirm and a keepalive is sent to the peer. Failure can result in sending the BGP state back to Idle or Active.
5. OpenConfirm  : neighbor replied with open message
The BGP state machine is one step away from reaching its final state (Established).
BGP waits in this state for keepalives from the peer. If successful, the state moves to Established; otherwise, the state moves back to Idle based on the errors.
6. Established : connection between neighbors established
This is the state in which BGP can exchange information between the peers. The information can be updates, keepalives, or notification.


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