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TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)
For ease of administration, routers
on large internetworks are grouped into autonomous systems (ASes)
that are independently controlled by different organizations and companies.
Interior
routing protocols such as RIP and OSPF
are used to communicate routing information between routers within an
autonomous system. Obviously, if interior routing protocols are used
within ASes, we need another set of routing protocols to send that information
between ASes. These are called exterior routing protocols.
The entire point of autonomous system
architecture is conveyed in the meaning of the first word in that phrase:
autonomous. The details of what happens within an AS are hidden
from other ASes, which allows the administrator of an AS to have the
independence to control how he or she runs it, including the selection
of one or more from a variety of different interior routing protocols.
In contrast, to reliably connect ASes together, it is essential that
each one be running the same exterior routing protocol, or the result
would be something akin to the Tower of Babel. The result of this is
that in TCP/IP there is generally only one exterior routing protocol
in widespread use at a given time.
In this section I describe two different
TCP/IP exterior routing protocols. The first is the Border Gateway
Protocol (BGP), the one used in modern TCP/IP. BGP is very important
since it is used on the current Internet and other larger internetworks,
so it is covered in considerable detail. The second is the Exterior
Gateway Protocol (EGP). This is an obsolete protocol that was used
for communication between non-core routers and the router core in the
early Internet, and is described briefly for both completeness and historical
interest.
 Background Information: I am assuming in this section that you are already at least somewhat familiar with interior routing protocols, at least to the extent of understanding what they do in basic terms. If you have not yet read up on RIP, the most common interior routing protocol, you may wish to skim that section. At the very least, make sure you are familiar with the overview of routing protocol architectures.
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