8.3 BGP Neighbor Negotiation
8.3.3 NOTIFICATION message
Since a NOTIFICATION message at any point can essentially close a peering relationship, network administrators will need to evaluate these NOTIFICATION messages to determine the specific error that caused the connection to reset. Figure illustrates the general message format.

The NOTIFICATION message is composed of the Error code (1 byte), Error subcode (1 byte), and a Data field (variable).

The Error code indicates the type of the notification, the Error subcode provides more specific information about the nature of the error, and the Data field contains data relevant to the error (such as a bad header or illegal AS number). Figure lists possible errors and their subcodes.