What are the BGP neighbor states?

• A. Idle, Connect, Active, Open Sent
• B. Idle, Connect, Active, Open Sent, Open Confirm and Established
• C. Idle, Active, Open
• D. Idle, Connect, Open Sent, Established

Answer: (B)

BGP neighbor states describe the lifecycle of a peering session, from not attempting a connection to fully exchanging routing information. The six states are Idle, Connect, Active, OpenSent, OpenConfirm, and Established.

In Idle, there’s no attempt to contact the neighbor. When a session is initiated, the state moves to Connect as a TCP connection is being established. If the TCP connection isn’t yet fully up, the process may be in Active, actively trying to reach the neighbor. Once the TCP session is up and a local OPEN message has been sent, the state becomes OpenSent. After the remote side’s OPEN is received and the neighbor exchange progresses with KEEPALIVE and possible UPDATE messages, the session enters OpenConfirm. When both sides have completed the OPEN/KEEPALIVE/UPDATE exchange, the session reaches Established, and typical BGP routing updates flow.

The other options omit one or more of these states or include a set that doesn’t correspond to the standard BGP FSM, so they don’t fully describe the neighbor lifecycle.