In BGP (Border Gateway Protocol), the Update message is used to advertise new routes to a peer or

to withdraw previously advertised routes. If there's a change in the routing information or if a new

route needs to be advertised, BGP uses an Update message. This message is crucial for maintaining

the most current and accurate routing information among BGP peers.

The other message types serve different purposes:

Keepalive: This message type is used to maintain the connection between BGP peers and ensure that

the link is still active. Keepalive messages are periodically sent between peers when there is no other

BGP activity to maintain the session.

Notification: This message is used to indicate errors or other significant events to a BGP peer. If a BGP

error condition is detected, a Notification message is sent and the BGP session is typically closed.

Refresh: This is not a standard BGP message type. However, in the context of BGP, Route Refresh is a

capability (not a message type) that allows a BGP router to request a full or partial re-advertisement

of routes from a peer. This helps in re-synchronizing routing tables without tearing down the BGP

session.

The BGP Update message is used to advertise routable destinations and includes route

advertisements, route withdrawals, and path attribute modifications. Update messages are used to

re-advertise routes that have already been acknowledged.

Reference::

BGP Operations and Message Types Overview, Juniper Networks Documentation

BGP Update Message, Juniper Networks Documentation