BGP Messages

BGP speakers exchange routing information with each other by exchanging BGP messages over a BGP session. BGP uses the following five message types:

• Open BGP messages—When two BGP speakers establish a BGP session with each other, the first message they exchange after the underlying TCP session has been established is an open message. This message contains various bits of information that enable the two BGP peers to determine whether they want to establish a BGP session with each other—for example, the AS number of the BGP speaker—and to negotiate certain parameters for the BGP session—for example, how often to send a keepalive message.
• Update messages—The update message is the most important message in the BGP protocol. A BGP speaker sends update messages to announce routes to prefixes that it can reach and to withdraw routes to prefixes that it can no longer reach.
• Keepalive messages—BGP speakers periodically exchange keepalive messages to determine whether the underlying TCP connection is still up.
• Notification messages—If a BGP speaker wishes to terminate a BGP session (either because it has been configured to do so or because it has detected some error condition), it will send a notification message to its peer specifying the reason for terminating the BGP session.

  If the session is being terminated for a nonfatal error, the notification messages includes the error code cease. Subcodes sent in the notification message can inform network operators about peering problems and help them better understand network events. Table 5 lists the subcodes defined for BGP notification messages bearing the cease code.

  Table 5: Cease Notification Message Subcodes

  Subcode	Reason	Symbolic Name
  1	The number of address prefixes received from the peer has exceeded the upper bound configured with the neighbor maximum-prefix command. The notification message can include address family and upper bound information in the data field.	Maximum Number of Prefixes Reached
  2	The BGP speaker is administratively shutting down the session.	Administratively Shutdown
  3	The BGP speaker is removing the peer configuration.	Peer Unconfigured
  4	The BGP speaker is administratively resetting the session.	Administratively Reset
  5	The BGP speaker is rejecting the connection (for example, because the peer is not configured locally on the speaker) after accepting a transport protocol connection.	Connection Rejected
  6	The BGP speaker is administratively resetting the session for some other configuration.	Other Configuration Change

• Route-refresh messages—BGP speakers can send route-refresh messages to peers that advertise the route-refresh capability. The messages contain a request for the peer to resend its routes to the router. This feature enables the BGP speaker to apply modified or new policies to the routes when it receives them again.

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