BGP Error Reporting: Notification Messages
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Once established, a BGP session will remain open for a considerable period of time, allowing routing information to be exchanged between devices on a regular basis. During the course of operation, certain error conditions may crop up that may interfere with normal communication between BGP peers. Some of these are serious enough that the BGP session must be terminated. When this occurs, the device detecting the error will inform its peer of the nature of the problem by sending it a BGP Notification message, and then close the connection.
Of course, having someone tell you I found an error, so I quit is not of much value. Therefore, the BGP Notification message contains a number of fields that provide information about the nature of the error that caused the message to be sent. This includes a set of primary error codes, as well as subcodes within some of these error codes. Depending on the nature of the error, an additional data field may also be included to aid in diagnosing the problem.
In addition to the use of Notification messages to convey the occurrence of an error, this message type is also used for other purposes. For example, one may be sent if two devices cannot agree on how to negotiate a session, which isn't, strictly speaking, an error. A Notification message is also used to allow a device to tear down a BGP session for reasons having nothing to do with an error.
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