BGP States

BGP States Mnemonic

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BGP States

Idle
Connect
Active
OpenSent
OpenConfirm
Established

Idle

–BGP detects a start event. –Tries to initiate a TCP connection with peer. –Listens for a new connection from peer.

Connect

–Initiates the TCP connection by sending a SYN packet: >Successful (receives SYN-ACK packet): sends Open message to neighbor and changes to OpenSent state. >Failure (doesn’t receive SYN-ACK): if ConnectRetry timer expires, it goes to Active state. –If any other input is received, state reverts to Idle.

Active

–Starts a new 3–way handshake, sending a SYN packet, but also listens for a SYN packet, as if passive: >Success: goes to OpenSent >Fail (doesn’t receive a SYN or SYN-ACK): resets ConnectRetry timer and returns to Connect.

OpenSent

Open message has been sent; awaiting Open message from peer. When received, checked for errors. 1. BGP identifiers (RIDs) must be unique. 2. Must match: –BGP versions. –Source IP and neighbor's IP. –AS #s. –Security parameters (PWs, TTL, etc.) must match. If errors detected, it will send a BGP Notification message to the neighbor indicating the error and go to Idle. If no errors, goes to OpenConfirm.

OpenConfirm

Awaits peer's Keepalive or Notification message. Receipt changes state to Established. If hold timer expires, state reverts to idle.

Established

Neighbors exchange routes via Update messages. Keepalive or update messages are received, resetting hold timers. Hold timer expiration reverts state to Idle.