The four BGP message types are open, keepalive, update, and notification.
Notification message: A BGP notification message is sent when an error condition is detected; the BGP connection is closed immediately after this is sent. Notification messages include an error code, an error subcode, and data that are related to the error.
A BGP peer must be configured with a BGP neighbor command. The administrator instructs the BGP speaker to establish a relationship with the address listed in the neighbor command and to exchange the BGP routing updates with that neighbor.
Recall that when BGP is running between routers in different autonomous systems, it is called EBGP.
Recall that BGP that runs between routers within the same autonomous system is called IBGP. IBGP runs within an autonomous system to exchange BGP information so that all BGP speakers have the same BGP routing information about outside autonomous systems.