tailieunhanh - Internetworking with TCP/IP- P32

Internetworking with TCP/IP- P32: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | 15 Flouting Exterior Gateway Protocols And Autonomous Systems BGP Introduction The previous chapter introduces the idea of route propagation and examines one protocol routers use to exchange routing information. This chapter extends our understanding of internet routing architectures. It discusses the concept of autonomous systems and shows a protocol that a group of networks and routers operating under one administrative authority uses to propagate routing information about its networks to other groups. Adding Complexity To The Architectural Model The original core routing system evolved at a time when the Internet had a single wide area backbone as the previous chapter describes. Consequently part of the motivation for a core architecture was to provide connections between a network at each site and the backbone. If an internet consists of only a single backbone plus a set of attached local area networks the core approach propagates all necessary routing information correctly. Because all routers attach to the wide area backbone network they can exchange all necessary routing information directly. Each router knows the single local network to which it attaches and propagates that information to the other routers. Each router learns about other destination networks from other routers. 269 270 Routing Exterior Gateway Protocols And Autonomous Systems BGP Chap. 15 It may seem that it would be possible to extend the core architecture to an arbitrary size internet merely by adding more sites each with a router connecting to the backbone. Unfortunately the scheme does not scale having all routers participate in a single routing protocol only suffices for trivial size internets. There are three reasons. First even if each site consists of a single network the scheme cannot accommodate an arbitrary number of sites because each additional router generates routing traffic. If a large set of routers attempt to communicate the total bandwidth becomes overwhelming.