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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 Fundamentals and General Operation

BGP Route Determination and the BGP Decision Process 1 2 BGP Detailed Messaging, Operation and Message Formats

Route Information Exchange

Assuming the link is initialized, the two peers begin an ongoing process of telling each other what they know about networks and how to reach them. Each BGP speaker encodes information from its Routing Information Bases (RIBs) into BGP Update messages. These messages contain lists of known network addresses, as well as information about paths to various networks, described in the form of path attributes. This information is then used for the route determination as described in the preceding topic.

When a link is first set up between two peers, they ensure complete information is held by each router by exchanging their complete routing tables. Subsequently, Update messages are sent that contain only incremental updates about routes that have changed. Exchanging only updated information as needed reduces unnecessary bandwidth on the network, making BGP more efficient than it would be if it sent full routing table information on a regular basis.

The TCP session between BGP speakers can be kept open for a very long time, but Updates need to be sent only when changes occur to routes, which are usually infrequent. This means many seconds may elapse between the transmission of Update messages. To ensure that the peers maintain contact with each other, they both send Keepalive messages on a regular basis when they don’t have other information to send. These are null messages that contain no data and just tell the peer device “I'm still here”. These messages are sent infrequently—no more often than one per second—but regularly enough that the peers won't think the session was interrupted.

The last type of BGP message is the BGP Notification message. This is an error message; it tells a peer that a problem occurred and describes the nature of the error condition. After sending a BGP Notification message, the device that sent it will terminate the BGP connection between the peers. A new connection will then need to be negotiated, possibly after the problem that led to the Notification has been corrected.

For more details on the four BGP message types and a description of the format used by each, refer to the detailed BGP operation section.

BGP Route Determination and the BGP Decision Process 1 2 BGP Detailed Messaging, Operation and Message Formats