order to transition to the OpenConfirm state. Keepalive messages are exchanged and, upon successful receipt, the router is placed into the Established state. In the Established state, the router can send and receive: Keepalive; Update; and Notification messages to and from its peer. Idle State: Refuse all incoming BGP connections. Start the initialization of event triggers. Initiates a TCP connection with its configured BGP peer. Listens for a TCP connection from its peer. Changes its state to Connect. If an error occurs at any state of the FSM process, the BGP session is terminated immediately and returned to the Idle state. Some of the reasons why a router does not progress from the Idle state are: TCP port 179 is not open. A random TCP port over 1023 is not open. Peer address configured incorrectly on either router. AS number configured incorrectly on either router. Connect State: Waits for successful TCP negotiation with peer. BGP does not spend much time in this state if the TCP session has been successfully established. Sends Open message to peer and changes state to OpenSent. If an error occurs, BGP moves to the Active state. Some reasons for the error are: TCP port 179 is not open. A random TCP port over 1023 is not open. Peer address configured incorrectly on either router. AS number configured incorrectly on either router. Active State: If the router was unable to establish a successful TCP session, then it ends up in the Active state. BGP FSM tries to restart another TCP session with the peer and, if successful, then it sends an Open message to the peer. If it is unsuccessful again, the FSM is reset to the Idle state. Repeated failures may result in a router cycling between the Idle and Active states. Some of the reasons for this include: TCP port 179 is not open. A random TCP port over 1023 is not open. BGP configuration error. Network congestion. Flapping network interface. OpenSent State: BGP FSM listens for an Open message from its peer. Once the message has been received, the router checks the validity of the Open message. If there is an error it is because one of the fields in the Open message does not match between the peers, e.g., BGP version mismatch, the peering router expects a different My AS, etc. The router then sends a Notification message to the peer indicating