Which two of the following are functions of the BGP update message? (Choose two.)

A.
It withdraws routes that are no longer valid.

B.
It notes that an unsupported option is detected from the open message.

C.
It transports routing information between BGP peers.

D.
It readvertises routes that have already been sent and acknowledged.

Explanation:
BGP uses the UPDATE message to send routing updates to peers. When a BGP session is initialized, UPDATE messages are sent until the complete BGP table has been exchanged.

Every time an UPDATE message is received, the BGP route table is updated and the BGP route table version number is incremented by one.

BGP advertises prefix and length sets and sends out one update message for each AS-path and attribute combination. For each AS-path, a set of NLRI announcements are sent in each update message.

An UPDATE message is used to advertise feasible routes that share common path attributes to a peer, or to withdraw multiple unfeasible routes from service (see 3.1). An UPDATE message MAY simultaneously advertise a feasible route and withdraw multiple unfeasible routes from service. The UPDATE message always includes the fixed-size BGP header, and also includes the other fields, as shown below (note, some of the shown fields may not be present in every UPDATE message):