What is a BGP Update message?

• A. Only Withdrawn Routes
• B. Advertises feasible routes, withdrawn routes and both. The Update message contains: NLRI, Path Attributes, and withdrawn routes or destination prefixes/lengths that have become unreachable.
• C. Contains Only NLRI
• D. Contains Open Messages and Keepalive Fields

Answer: (B)

A BGP Update message is how two peers tell each other about changes in reachability. It can carry two kinds of information: routes that are being withdrawn and routes that are being announced or updated. The Update payload is structured to include the withdrawn routes first, then the Path Attributes that describe how to reach those routes, and finally the NLRI for the routes being advertised. If there are withdrawals, they appear in the Withdrawn Routes section; if there are new or updated routes, their prefixes appear in the NLRI section along with the necessary path attributes. It can do both in one message, or contain only withdrawals or only announcements if needed. Open messages and Keepalive messages are separate types used to manage the session, not Update messages. So the description that an Update message advertises feasible routes, withdrawn routes and both, and contains NLRI, Path Attributes, and withdrawn routes, is the best fit.