BGP – Finite State Machine

IDLE State

  • BGP Always beging with Idle state. All incoming connection in this state will be refused.
  • BGP Process initializes all BGP resources.
  • Start the ConnectRetry Timer
  • Initializes a TCP connection to neighbor
  • Listen for TCP initialization from neighbor.
  •  And changes its state to CONNECT.

CONNECT State

  • Waits for TCP connection to complete.
  • If TCP conn successful, it will clear ConnectRetry Timer.
  • Completes Initialization and sends a OPEN Message to neighbor.
  • And changes the state to OPENSENT State.

ACTIVE State

  • Keep on trying to initialize a  TCP connection to complete neighbor.
  • If TCP conn successful, it will clear ConnectRetry Timer.
  • Completes Initialization and sends a OPEN Message to neighbor.
  • And changes the state to OPENSENT State.

OPENSENT State

  • Send OPEN Message and Wait for a OPEN Message from neighbor too.
  • Once OPEN received its verifies the fields.
  • If any error, sent NOTIFICATION and change to IDLE state.
  • If no Error, a KEEPALIVE message is sent and keepalive timer is reset.
  • HOLD timer is negotiated by selecting the least value.
  • If negotiated HOLD is zero, keepalive and hold timer are not started.
  • Determines whether eBGP or iBGP.
  • Change the state to OPENCONFIRM.

 OPENCONFIRM State

  • Waits for either a Keepalive or Notification Message.
  • If Keepalive, change the state to ESTABLISHED state
  • If Notification,  or a TCP disconnect, or HOLD timer expires change to IDLE state.

ESTABLISHED State

  • BGP peer connection is fully established and BGP routers can exchange their UPDATE/KEEPALIVE/NOTIFICATION messages.
  • If UPDATE or KEEPALIVE Hold Timer is restarted.
  • If NOTIFICATION, changed to IDLE state.

 

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