Computer Networking: Internet Protocols in Action

Chapter 4.4. Border Gateway Protocol

INTRODUCTION

In Exercise 4.3, we explored dynamic routing protocols. We saw that in distance vector routing protocols, news of available paths spreads through the network as neighboring routers exchange information about the best paths to each destination in the network. We also discussed that in link-state routing protocols, each router sends information about the state of their local links to every other router allowing each router to build a full picture of the entire network.

Now consider what would happen if we used either a distance vector or link-state protocol to exchange information on every route in the Internet. In the case of link-state, an announcement would be sent to every router each time a link changed state anywhere in the Internet! In the case of distance vector, neighboring routers would be periodically exchanging their best-known path to every destination! In either case, the dynamic routing protocol messages would gobble up much of the available bandwidth.

The primary strategy for avoiding these problems is "divide and conquer." The Internet as a whole is first divided up into smaller networks called Autonomous Systems (AS). Routing within each AS is hidden from the rest of the Internet and then routing is done between ASes. For example, a university AS may have many routers and many sub networks. Within the university, these routers will communicate with each other to determine the best path to each sub network. However, when ...