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Autonomous Systems and Exterior Routing
Routing between autonomous systems or exterior routing is strictly
an Internet convention. What was once a practice undertaken by a
chosen few is now (with more and more networks connecting to the
Internet) becoming a commonplace activity. ERPs were created to
provide a more efficient way to build and manage Internet backbone
routing tables. The idea was to segment the Internet into separate
"ubernetworks" called autonomous systems (ASs). An AS is,
according to RFC 1267, A Border Gateway Protocol 3 (BGP-3),
Lougheed and Rekhter, Network Working Group 1991:
"A set of routers under a single technical administration, using an
interior gateway protocol and common metrics to route packets within
the AS and using an exterior gateway protocol to route packets to
other autonomous systems."
Coincidentally, this idea has translated well into the way the modern
global Internet has evolved.
NOTE
It is common to confuse an internetwork and an AS because the terms
are often used interchangeably. The difference is that an internetwork
uses one or more IGP protocols to exchange routing information
within a closed network. An AS uses both IGP(s) and ERPs to
exchange routing information within the context of an open network
(like the Internet). In addition, an AS network is registered with an
Internet registration authority (ICANN, AIRN, or RIPE).
There are three types of ASs:
• Stub—Stub ASs reach other networks through one gateway.
Routing for stub AS is commonly achieved with static routes.
Stub ASs are uncommon today, because most IP addresses are
provided by ISPs and NSPs, and those addresses fall under their
own AS.
• Multihomed nontransit—Large private enterprise networks,
with multiple Internet access points, commonly operate as ASs.
Because these networks operate their own network backbone
infrastructures, they do not want to have traffic other than their