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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 Topology, Speakers, Border Routers and Neighbor Relationships (Internal and External Peers) 1 2 3 BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)

Issues With Routing Policies and Internetwork Design

What would happen to a city if every street only allowed local traffic? It would be pretty hard to get around. Of course this problem never occurs in well-designed cities, because traffic planners understand the dual need for connectivity and through-traffic avoidance in residential areas. Cities are laid out in a somewhat hierarchical fashion, so local traffic “funnels” to thoroughfares intended specifically to carry non-local traffic.

The same basic situation exists in an internetwork. It wouldn't work very well if every AS declared it was not interested in carrying transit traffic! Usually, internetworks are designed so that certain autonomous systems are intended to carry large amounts of transit traffic. This is typically the function of high-speed, high-capacity backbone connections, which serve other ASes as customers. An AS will usually only carry another AS's traffic if arrangements have been made to allow this.

BGP Topology, Speakers, Border Routers and Neighbor Relationships (Internal and External Peers) 1 2 3 BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)