BGP Finite State Machine Operation

BGP peers transition through several states before becoming adjacent neighbors and exchanging routing information. During each of the states, the peers must send and receive messages, process message data, and initialize resources before proceeding to the next state. This process is known as the BGP Finite-State Machine (FSM) . If the process fails at any point, the session is torn down and the peers both transition back to an Idle state and begin the process again. Each time a session is torn down, all routes from the peer who is not up will be removed from the tables, whic hi causes downtime. If configuration issues exist on one of the BGP peers, the peering Bouters continuously transition betwE en unestablished states until the issue has been resolved. BGP peers transition through all the following states until an established BGP session has been created:

• Opennoniirrtl

• Established

Each of these s tates also has accompanying input events (IEs). Input events are events occurring during a BGP session that trigger an action. Table 7-11 shows the BGP IEs.

Table 7-11. BGP Input Events

Ev ent ID

E vent Name

D escri ption

1

BGP Start

Occurring during the Idle state, the BGP start event signals the beginnm g of a BGP session. It also initializes the tesou rcbs for the BGP process. The BGP start event is only listened nor during the Idle state. If th e loces speaker receives a htart even t a nd it is not in the I dle staoe, thnt event is ¡gr^ed.

n

BGP S top

The BGP Stop signals the tgrmi nation of a BG P mes^n i

33

BGP Transport Connection Open

This eues t notifies the local speaker that the TCP connection is open and the BGP resource initialization is complete.

Connection

Closed

This event notifies the local speaker that the remote BGP speaker has closed the TCP session. This message also triggers the release of BGP resources and causes the local speaker to return to the Idle state.

Connection

Failed

This event notifies the local speaker that the TAP session to the remote BGP peer has failed. This message also triggers the release of BGP resources and causes the local speaker to return to the Idle state.

6

BGP Transport Fatal Error

This event notifies the local speaker that the TAP session to the remote BGP peer has resulted in a fatal error. This message also triggers the release of BGP resources and causes the local speaker to return to the Idle state.

7

ConnectRetry Timer Expired

This event occurs when the ConnectRetry timer has expired. When the ConnectRetry timer expires, it is restarted.

8

Hold Timer Expired

TNs even t occurs when the hold timer has expired, meaning that the remote peer has not responded to a message from the local peer.

9

KEEPALIVE Ti mer Expired

This event indicates that the KEEPALIVE timer expired, signaling that a KEEPALIVE has not been received from the remote peer within the timeout period.

10

Receive Open Message

This event notifies the local system that a BGP OPEN message has been received by the remote peer, and the BGP session can move on to the OpenConfirm state.

KEEPALIVE

Message

This event notifies the loca l system tha t a BG P KEEPALWE message has been received by the remote peer, and the BGP session can move on to the Established state.

12

Receive Update Message

This event notifier the; loaal system that the remote peer has received a BGP UPDATE message.

Notification

Message

This event notifies the local system that a BGP NOTIFICATION message has been received, and the BGP session should be terminaned immediately.

Was this article helpful?

0 0

Post a comment