Which of the following messages are sent when BGP establishes peer relationships?

Comprehensive and Detailed in-depth Step-by-Step Explanation:

BGP (Border Gateway Protocol) uses specific message types to establish and maintain peer relationships between routers. The primary BGP messages involved during the establishment and maintenance of peer relationships are:

Open (D):

The Open message is the first message sent after a TCP connection is established between BGP peers.

It is used to exchange parameters such as the AS number, BGP version, hold time, and BGP identifier.

Keepalive (A):

After the Open message exchange, Keepalive messages are sent periodically to maintain the BGP session.

These messages confirm that the connection is still alive.

Update (C):

Update messages are used to exchange routing information after the peer relationship is established.

They contain information about network prefixes and their associated path attributes.

Hello (B):

Hello messages are not used in BGP. They are typically used in protocols like OSPF or IS-IS to establish and maintain neighbor relationships.

Thus, the correct answers are A (Keepalive) , C (Update) , and D (Open) .