Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)                      9  TCP/IP Border Gateway Protocol (BGP/BGP-4)                           9  BGP Detailed Messaging, Operation and Message Formats

BGP Connectivity Maintenance: Keepalive Messages 1 2 3 TCP/IP Exterior Gateway Protocol (EGP)

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.

BGP Connectivity Maintenance: Keepalive Messages 1 2 3 TCP/IP Exterior Gateway Protocol (EGP)