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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 Storage and Advertisement, and BGP Routing Information Bases (RIBs) 1 2 3 BGP Route Determination and the BGP Decision Process

Routing protocols using a distance-vector algorithm such as RIP are relatively simple in large part because the information each device stores about each route is itself simple. Each router only knows that it can reach a network at a specific cost through a particular next-hop router. It doesn't have knowledge of the route that datagrams will take to reach any of these networks. This level of knowledge is simply insufficient for the needs of a protocol like BGP.

In order to handle the calculation of efficient, non-looping routes in an arbitrary topology of autonomous systems, we need to know not just “to get to network N7 send to router R4”; we need to have some understanding of the characteristics of the entire path between ourselves and network N7. By storing this additional information, it is possible to make decisions about how to compute and change routes, using knowledge of the entire path between a router and a network.

Thus, in BGP, instead of advertising networks in terms of a destination and the distance to that destination, BGP devices advertise networks as destination addresses and path descriptions to reach those destinations. This means BGP uses, instead of a distance-vector algorithm, a path-vector algorithm. Each communication of a reachable network provides considerable information about the entire sequence of routers to a destination. Due to this inclusion of topology information, path-vector protocols are sometimes described as a combination of distance-vector and link-state algorithms. This doesn't really do them justice, however, since they do not function in the same way as either of those algorithm types.

The information about the path to each route is stored in the Routing Information Base (RIB) of each BGP speaker in the form of BGP path attributes. These attributes are used to advertise routes to networks when BGP devices send out Update messages. The storing, processing, sending and receiving of path attributes is “the” method by which routers decide how to create routes, so understanding them is obviously quite important.

There are several different path attributes, each of which describes a particular characteristic of a route. Attributes are divided into different categories based on their level of importance and specific rules designed to manage their propagation. The most important path attributes are called well-known attributes; every BGP speaker must recognize and process these, but only some are required to be sent with every route. Other attributes are optional and may or not be implemented. These are further differentiated based on how they are handled when received by a device that does not recognize them.

BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs) 1 2 3 BGP Route Determination and the BGP Decision Process