Foundation Topics
Like Interior Gateway Protocols (IGPs), BGP exchanges topology information in order for routers to eventually learn the best routes to a set of IP prefixes. Unlike IGPs, BGP does not use a metric to select the best route among alternate routes to the same destination. Instead, BGP uses several BGP path attributes (PAs) and an involved decision process when choosing between multiple possible routes to the same subnet.
BGP uses the BGP autonomous system path (AS_PATH) PA as its default metric mechanism when none of the other PAs has been overly set and configured. Generally speaking, BGP uses PAs to describe the characteristics of a route; both this chapter and Chapter 13 cover the wide variety of BGP PAs. The AS_PATH attribute lists the path, as defined by a sequence of autonomous system numbers (ASNs) through which a packet must pass to reach a prefix. Figure 12-1 shows an example.
Figure 12-1 BGP AS PATHs and Path Vector Logic
ASN2 ASN 3
ASN2 ASN 3
Figure 12-1 shows a classic case of how BGP uses path vector logic to choose routes. In the figure, R1 learns of two AS_PATHs by which to reach 9.0.0.0/8—through ASNs 2-3 and through ASNs 5-4-3. If none of the routers has used routing policies to influence other PAs that influence BGP's choice of which route is best, R1 will choose the shortest AS_PATH—in this case, AS_PATH 2-3. In effect, BGP treats the AS_PATH as a vector, and the length of the vector (the number of ASNs in the path) determines the best route. With BGP, the term route still refers to traditional hop-by-hop IP routes, but the term path refers to the sequence of autonomous systems used to reach a particular destination.
Figure 12-1 shows a classic case of how BGP uses path vector logic to choose routes. In the figure, R1 learns of two AS_PATHs by which to reach 9.0.0.0/8—through ASNs 2-3 and through ASNs 5-4-3. If none of the routers has used routing policies to influence other PAs that influence BGP's choice of which route is best, R1 will choose the shortest AS_PATH—in this case, AS_PATH 2-3. In effect, BGP treats the AS_PATH as a vector, and the length of the vector (the number of ASNs in the path) determines the best route. With BGP, the term route still refers to traditional hop-by-hop IP routes, but the term path refers to the sequence of autonomous systems used to reach a particular destination.
This chapter follows a similar sequence as several of the IGP chapters. First, the text focuses on neighbor relationships, followed by how BGP exchanges routing information with its neighbors. The chapter ends with a section covering how BGP adds IP routes to a router's IP routing table based on the BGP topology table.
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