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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

In my overview of BGP, I boiled down the function of the Border Gateway Protocol (BGP) into this summary: the exchange of network reachability information between autonomous systems (ASes) of routers and networks, and the determination of routes from this information. The actual method that BGP uses to accomplish this, however, is fairly complex. To help us understand how BGP works, we should start by looking at the structure of BGP internetworks, and also a discussion of key BGP terms.

One of the most important characteristics of BGP is its flexibility. The protocol can connect together any internetwork of autonomous systems using an arbitrary topology. The only requirement is that each AS have at least one router that is able to run BGP and that this router connect to at least one other AS's BGP router. Beyond that, “the sky's the limit,” as they say. BGP can handle a set of ASes connected in a full mesh topology (each AS to each other AS), a partial mesh, a chain of ASes linked one to the next, or any other configuration. It also handles changes to topology that may occur over time.

Another important assumption that BGP makes is that it doesn't know anything about what happens within the AS. This is of course an important prerequisite to the notion of an AS being autonomous—it has its own internal topology and uses its own choice of routing protocols to determine routes. BGP only takes the information conveyed to it from the AS and shares it with other ASes.

Creating a BGP internetwork begins with the designation of certain routers in each AS as ones that will run the protocol. In BGP parlance, these are called BGP speakers, since they speak the BGP “language”. A protocol can reasonably be called a language, but I have not encountered this notion of a “speaker” in any other protocol, so it's somewhat interesting terminology.

An autonomous system can contain many routers which are connected in an arbitrary topology. We can draw a distinction between routers in an AS that are connected only to other routers within the AS, versus those that connect to other ASes. Routers in the former group are usually called internal routers, while the latter group are called border routers in BGP, and similar names in other protocols. For example, in OSPF they are called boundary routers.

Of course, the notion of a border is the basis for the name of the BGP protocol itself. To actually create the BGP internetwork, the BGP speakers bordering each AS are physically connected to one or more BGP speakers in other ASes, in whatever topology the internetwork designer decrees. When a BGP speaker in one AS is linked to a BGP speaker in another AS, they are deemed neighbors. The direct connection between them allows them to exchange information about the ASes of which they are a part.

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