8.3 BGP Neighbor Negotiation
8.3.2 Finite state machine perspective example
BGP neighbor negotiation proceeds through different stages before the connection is fully established. The Figure illustrates a simplified finite-state machine (FSM) that highlights the major events in the process with an indication of messages (OPEN, KEEPALIVE, NOTIFICATION) sent to the peer in the transition from one state to the other.

The following discussions summarize the key states in the FSM example illustrated in the Figure:

  • Idle -- This is the first stage of the connection. BGP is waiting for a Start event, which is normally initiated by an operator or a network event; a Start event is usually caused by an administrator configuring a BGP session, resetting an already existing session, or a link coming up across which a BGP is configured. At the Start event, BGP initializes its resources, resets a connect retry timer, initiates a TCP transport connection, and starts listening for a connection that may be initiated by a remote peer. BGP then transitions to a Connect state. In case of errors, BGP falls back to the Idle state.
  • Connect -- BGP is waiting for the transport protocol connection to be completed. If the TCP transport connection is successful, the state transitions to OpenSent (this is where the OPEN message is sent). If the transport connection is not successful, the state transitions to Active. If the connect retry timer expires, the state will remain in the connect stage, the timer will be reset, and a transport connection will be initiated. In case of any other event (initiated by system or operator), the state will go back to Idle.
  • Active -- BGP is trying to acquire a peer by initiating a transport protocol connection. If it is successful, it will transition to OpenSent (an OPEN message is sent). If the connect retry timer expires, BGP will restart the connect timer and fall back to the connect state. While Active, BGP is still listening for a connection that may be initiated from another peer. The state may go back to Idle in case of other events, such as a stop event initiated by the system or the operator.

In general, a neighbor state that is flip-flopping between Connect and Active is an indication that something is wrong, and the TCP transport connection is not taking effect. It could be because of many TCP retransmissions or the inability of a neighbor to reach the IP address of its peer.

  • OpenSent -- BGP is waiting for an OPEN message from its peer. The OPEN message is checked for correctness. In case of errors, such as a bad version number or an unacceptable AS, the system sends an error NOTIFICATION message and goes back to Idle. If there are no errors, BGP starts sending KEEPALIVE messages and resets the KEEPALIVE timer. At this stage, the hold time is negotiated, and the smaller value is taken. If the negotiated hold time is 0, the hold timer and the KEEPALIVE timer are not restarted.

At the OpenSent state, the BGP will recognize whether the peer belongs to the same AS (IBGP) or to a different AS (EBGP) by comparing its AS number to the AS number of its peer.

When a TCP transport disconnect is detected, the state will fall back to Active. For any other errors, such as an expiration of the hold timer, BGP will send a NOTIFICATION message with the corresponding error code and will fall back to the Idle state.

  • OpenConfirm -- While in this state, BGP is waiting for a KEEPALIVE or NOTIFICATION message. If a KEEPALIVE is received, the state will go to established, and the neighbor negotiation is complete. If the system receives an UPDATE or KEEPALIVE message, it restarts the hold timer (assuming that the negotiated hold time is not 0). If a NOTIFICATION message is received, the state falls back to Idle. The system will send periodic KEEPALIVE messages at the rate set by the KEEPALIVE timer. In case of any transport disconnect notification or in response to any stop event (initiated by the system or the operator), the state will fall back to Idle. In response to any other event, the system will send a NOTIFICATION message with an FSM error code and will go back to Idle.
  • Established -- This is the final stage in the neighbor negotiation; BGP starts exchanging UPDATE packets with its peers. Assuming that it is non-zero, the hold timer is restarted at the receipt of an UPDATE or KEEPALIVE message.

Each UPDATE message is checked for errors, such as missing or duplicate attributes, etc; if errors are found, a NOTIFICATION is sent to the peer. Any NOTIFICATION received while Established will cause the BGP process to drop this peer back to Idle. If the hold timer expires, a disconnect notification is received from the transport protocol, or a Stop event is received, then the system will fall back to Idle.