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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.)
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Version 3.0 - Version Date: September 20, 2005
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