Which of the following is NOT a BGP message type?

BGP (Border Gateway Protocol) uses several specific message types to facilitate communication between peers. The message types defined in the BGP specification are Keepalive, Update, and Notification messages.

Keepalive messages are sent periodically to confirm that a BGP connection is still active and to maintain the session. Update messages convey routing information, including path attributes and withdrawn routes. Notification messages are used to communicate errors or provide status updates regarding the BGP session, indicating issues such as changes in connection states.

The Request message, however, is not a defined message type within the BGP protocol. This omission means that while BGP has mechanisms for request-like behavior through other means, such as session negotiation, it does not specifically recognize a "Request message" as part of its defined communication structure. This makes the Request message the correct answer as the one that is NOT a BGP message type.