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Chapter 7
BGP's Basic Components
RFC 1930 defines an autonomous system as a set of routers under one or
more administrations that presents a common routing policy to the internet
(lowercase i). By definition, an internet is a set of interconnected networks that
cooperate with each other, advertise policies, and contain a specified level of
independence. There are many organizations, such as the government, state
departments, and financial institutions, with networks large enough to need
BGP and to split into multiple autonomous systems.
Inside autonomous networks, interior routing protocols called interior
gateway protocols (IGP) are used to discover the connectivity among a set of
IP subnets. IGPs are well-known protocols such as the Routing Information
Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Open Shortest
Path First (OSPF), and Enhanced Interior Gateway Routing Protocol
(EIGRP). In Figure 7.1, we see an example of two corporate networks being
connected by BGP.
F I G U R E 7 . 1
Two corporate networks being connected by BGP
Company A
Autonomous
System 204
Company B
Autonomous
System 51,064
Autonomous
System 64,042
Customers all in
their own
Autonomous
Systems
ISP
Internet
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