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Table Of Contents  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

Previous Topic/Section
BGP Topology, Speakers, Border Routers and Neighbor Relationships (Internal and External Peers)
Previous Page
Pages in Current Topic/Section
12
3
Next Page
BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)
Next Topic/Section

BGP Autonomous System Types, Traffic Flows and Routing Policies
(Page 3 of 3)

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.

Key Concept: One important issue in BGP is how to handle the flow of traffic between autonomous systems. Each autonomous system in a BGP internetwork is either a stub AS, if it connects to only one other AS, or a multihomed AS if it connects to two or more others. BGP allows the administrators of a multihomed AS to establish routing policies that specify under what conditions the AS is willing to handle transit traffic (messages sent over the AS whose source and destination are both external to that AS.)



Previous Topic/Section
BGP Topology, Speakers, Border Routers and Neighbor Relationships (Internal and External Peers)
Previous Page
Pages in Current Topic/Section
12
3
Next Page
BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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