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 Connection Establishment: Open Messages 1 2 BGP Connectivity Maintenance: Keepalive Messages

Once BGP speakers have made contact and a link has been established using Open messages, the devices begin the actual process of exchanging routing information. Each BGP router uses the BGP Decision Process to select certain routes to be advertised to its peer. This information is then placed into BGP Update messages, which are sent to every BGP device for which a session has been established. These messages are the way that network reachability knowledge is propagated around the internetwork.

Each Update message contains either one or both of the following:

• Route Advertisement: The characteristics of a single route.
  
  
• Route Withdrawal: A list of networks that are no longer reachable.

Only one route can be advertised in an Update message, but several can be withdrawn. This is because withdrawing a route is simple; it requires simply the address of the network for which the route is being removed. In contrast, a route advertisement requires a fairly complex set of path attributes to be described, which takes up a significant amount of space. (Note that it is possible for an Update to only specify withdrawn routes and not advertise a route at all.)

BGP Connection Establishment: Open Messages 1 2 BGP Connectivity Maintenance: Keepalive Messages