What is the purpose of a BGP Open message?

The BGP Open message serves as the initial communication between BGP peers to establish a session. When a router wants to initiate a BGP connection with another router, it sends an Open message, which includes important information such as the BGP version being used, the AS (Autonomous System) number, the BGP Router ID, and various optional parameters. This message is critical for establishing a successful BGP session and sets the groundwork for future exchanges of route information.

Once the Open message is sent, the receiving peer must acknowledge it with a Keepalive message to confirm that the session is established. Until this Open message is successfully exchanged and acknowledged, the BGP session is not considered active. The fundamental role of the Open message in initiating communication distinguishes it as a key component of BGP’s operation when routers begin to form peer relationships.