BGP Basic- State

Idle State

BGP always begins in the Idle state, in which it refuses all incoming connections. When a Start event

, the BGP process initializes all BGP resources, starts the ConnectRetry timer, initializes

a TCP connection to the neighbor, listens for a TCP initialization from the neighbor, and changes its

state to Connect.

Connect State

In this state, the BGP process is waiting for the TCP connection to be completed. If the TCP

connection is successful, the BGP process clears the ConnectRetry timer, completes initialization,

sends an Open message to the neighbor, and transitions to the OpenSent state.,

If the ConnectRetry timer expires while in the Connect state, the timer is reset, another attempt is

made to establish a TCP connection with the neighbor, and the process stays in the Connect state.

Active State

In this state, the BGP process is trying to initiate a TCP connection with the neighbor. If the TCP

connection is successful, the BGP process clears the ConnectRetry timer, completes initialization,

sends an Open message to the neighbor, and transitions to OpenSent. The Hold timer is set to 4

minutes.

If the ConnectRetry timer expires while BGP is in the Active state, the process transitions back to the

Connect state and resets the ConnectRetry timer.

OpenSent State

In this state, an Open message has been sent, and BGP is waiting to hear an Open from its neighbor.

When an Open message is received, all its fields are checked. If errors exist, a Notification message is sent and the state transitions to Idle.

If no errors exist in the received Open message, a Keepalive message is sent and the Keepalive timer

is set.

OpenConfirm State

In this state, the BGP process waits for a Keepalive or Notification message. If a Keepalive is

received, the state transitions to Established. If a Notification is received, or a TCP disconnect is

received, the state transitions to Idle.

Established State

In this state, the BGP peer connection is fully established and the peers can exchange Update,

Keepalive, and Notification messages