BGP Connection Establishment: Open Messages
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Before a BGP session can be used to exchange routing information, a connection must first be established between BGP peers. This process begins with the creation of a TCP connection between the devices. Once this is done, the BGP devices will attempt to create a BGP session by exchanging BGP Open messages.
The Open message has two main purposes. The first is identification and initiation of a link between the two devices; it allows one peer to tell the other I am a BGP speaker named X on autonomous system Y, and I want to start exchanging BGP information with you. The second is negotiation of session parameters. These are the terms by which the BGP session will be conducted. One important parameter negotiated using Open messages is the method that each device wants to use for authentication. The importance of BGP means that authentication is essential, to avoid bad information or a malicious person from disrupting routes.
Each BGP receiving an Open message processes it. If its contents are acceptable, including the parameters the other device wants to use, it responds with a Keepalive message as an acknowledgment. Each peer must send an Open and receive a Keepalive acknowledgment for the BGP link to be initialized. If either is not willing to accept the terms of the Open, the link is not established. In that case, a Notification message may be sent to convey the nature of the problem.
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