IP Routing

Chapter 7. Border Gateway Protocol 4 (BGP-4)

My first passport, issued by the Government of India in 1981, bore a curious stamp in bold red ink on one of its first pages: “NOT VALID FOR TRAVEL IN THE REPUBLIC OF SOUTH AFRICA.”

Toward the end of the 1990s, I was traveling through Europe with a U.S. passport. Upon landing at Schiphol Airport in Amsterdam, I was stopped by an officer and led to a small room off to the side. There, I was made to remove my left shoe and sock. The officer checked my sock carefully before letting me go. Once I had cleared Dutch customs, I freely roamed up and down that beautiful country, walking barefoot over grass and flowers, past windmills.

Nations have policies concerning who can pass through their borders. So, India did not permit its citizens to grace the Republic of South Africa during its reign of apartheid. And the Dutch bar entry into their country if your left sock is not fresh and clean.

What does all this have to do with internetworking, or BGP-4, for that matter? Each network is an autonomous system (AS) managed by a single technical entity and under one political administration. ASs are akin to nation states. Much as nation states apply their immigration policies at international airports, seaports, and land border points, the Internet is composed of ASs that use Border Gateway Protocol 4 (BGP-4) to implement inter-AS IP routing policies.

What is a routing policy? Consider the topology in Figure 7-1. TraderMary has two links to the Internet: ...