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Autonomous Systems

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BGP update messages

BGP has been used for quite some time on routers connecting to the Inter-

net. The Internet can be really thought of as the backbone of thousands of
small and large companies. This book focuses on the latest version of BGP:
BGP version 4(BGPv4). BGPv4 is an exterior routing protocol. Interior rout-
ing protocols such as RIP, IGRP, and OSPF run inside a company's network.
BGP is the glue that connects the different networks to the Internet. BGP also
helps in finding and distributing route information.

Are you ready for all of this? Let's get started and see what an autono-

mous system is.

Autonomous Systems

ou can imagine the Internet as a Lego castle. In order to build a

LegoTM castle, you need many pieces. The same goes for the Internet. The
Internet is built with many autonomous systems, which we will think of as
Lego pieces. These pieces are then assembled to form a much larger piece.
Autonomous systems (AS) are the basic building blocks of network-to-
network routing. An autonomous system can be the entire corporate net-
work comprised of multiple locations connecting to the network.

An AS uses BGP to advertise routes in its network that need to be visible

outside of the network; it also uses BGP to learn about the reachability and
routes by listening to advertisement announcements from other autonomous
systems. Each AS can have a specific policy regarding the routes it wishes to
advertise externally. These policies can be different for every point in which
the AS attaches to the outside world.

The Internet consists of a number of commercial networks that connect to

each other via tier-one providers, such as Sprint, Qwest, WorldCom/MCI,
UUNet, and many others. Each enterprise network or ISP must be identified
by an autonomous system number (ASN). This number allows a hierarchy to
be maintained when sharing route information.

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