Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 21
IGPs, EGPs, and Routing Protocols

There are two classifications of propagating information: Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP). An IGP is a routing protocol that propagates information inside one autonomous system. An EGP is a routing protocol that propagates information between autonomous systems.

In order for data to be moved across an internet, information on the location of the networks must be propagated throughout the network. This is the introduction to the difference between a routing protocol and a routable protocol. IP is a routable protocol. Propagating information throughout the network as to the location of the networks is known as a routing protocol. Don’t confuse the two.

I know that I keep using the term autonomous system (AS). Yes, it is defined as a network that is under a single administrative control, but let’s define that a little—and yes, it does get a little blurry. Before the plethora of ISPs, anyone connected to the Internet was assigned an address and used a special protocol (then known as EGP) to connect to the Internet. Therefore, that connection became known as an autonomous system, and routes for that network were known on the Internet using EGP (yes, the acronym for the protocol is the same one used for the definition of the protocol). Autonomous systems were simply entities connected to the Internet. They were given a special AS number, and EGP knew how to route this data. An AS could mean a four–user office with a single Internet connection, a network as large as the one used by General Motors, or an Internet Service Provider (ISP). So don’t get confused by the term autonomous system.

Today, ISPs rule the connection to the Internet and an AS is more blurry. The new protocol that controls routes on the Internet is known as Border Gateway Protocol (BGP), and it is an EGP (as opposed to an IGP). However, only certain ISPs need this protocol; all others are simply connections (hierarchical) off of their upstream ISP. So AS takes on a new meaning. For our purposes, yes, it still means a single customer network, but for the Internet, it is generally the upper–end ISP. Many IP networks are simply running as part of their ISP AS.