Open message

After a TCP connection is established between two BGP routers the first message sent by each one is an Open message. If the received Open message is acceptable a Keepalive message confirming the Open is sent back. For more information, see BGP session states.

An Open message contains the following information:

• Version

  The current BGP version number is 4.

• Autonomous system number

  The 2-byte AS of the sending router. If the sending router has an ASN greater than 65535, this field has the special value 23456 (AS_TRANS). On a 7450, 7750, or 7950 router, the ASN in the Open message is based on the confederation ID (if the peer is external to the confederation), the global AS (configured using the autonomous-system command) or a session-level override of the global AS called the local AS (configured using the local-as command). More details about the use of local-AS are described in the section titled Using local AS for ASN migration. More details about 4-byte AS numbers are described in the section titled 4-octet autonomous system numbers.

• Hold time

  The proposed maximum time BGP waits between successive messages (Keepalive and/or Update) from its peer before closing the connection. The actual hold time is the minimum of the configured hold-time for the session and the hold-time in the peer's Open message. If this minimum is below a configured threshold (min hold-time), the connection attempt is rejected.

  Note: Changes to the configured hold-time trigger a session reset.

• BGP identifier

  The router ID of the BGP speaker. In Open messages, the BGP Identifier comes from the router-id configured under bgp; if that is not configured, then the router-id configured under config>router (or config>service>vprn) is used and if that too is not configured then the system interface IPv4 address is used.

  Note: A change of the router ID in the config>router>bgp context causes all BGP sessions to be reset immediately while other changes resulting in a new BGP identifier only take effect after BGP is shutdown and re-enabled.

• Optional parameters

  A list of optional parameters, each encoded as a TLV. The only optional parameter that has been defined is the optional parameter. The optional parameter supports the process of BGP advertisement (see BGP advertisement for more information). When a BGP router receives an Open message with an unsupported optional parameter type it terminates the session. Unless disable-capability-negotiation is configured, the router always sends an optional parameter in its Open message.