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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 Overview, History, Standards and Versions 1 2 BGP Autonomous System Types, Traffic Flows and Routing Policies

BGP Internal and External Peers

Most BGP speakers will be connected to more than one other speaker. This provides both greater efficiency in the form of more direct paths to different networks, and also redundancy to allow the internetwork to cope with either device or connection failures. It is possible (and in many cases, likely) for a BGP speaker to have neighbor relationships with other BGP speakers both within its own AS and outside its AS. A neighbor within the AS is called an internal peer, while a neighbor outside the AS is an external peer. BGP between internal peers is sometimes called Internal BGP (IBGP) while use of the protocol between external peers is External BGP (EBGP). The two are similar, but differ in certain areas, especially path attributes and route selection. You can see an example of BGP topology and the designation of internal and external peers in Figure 191.

A peer connection between BGP speakers can be either a direct link using some form of layer two technology, or an indirect link using TCP. This allows the BGP speakers to establish BGP sessions and then exchange routing information, using the messaging system we will see later in this section. Of course, it also is the means by which actual “end user” traffic moves between autonomous systems. External peers are normally connected directly, while internal peers are often linked indirectly.

In the next topic we will see that the method in which ASes are connected has an important impact on the overall function of the internetwork and how traffic is carried on it.

BGP Overview, History, Standards and Versions 1 2 BGP Autonomous System Types, Traffic Flows and Routing Policies