Site home page
(news and notices)

Get alerts when Linktionary is updated

Book updates and addendums

Get info about the Encyclopedia of Networking and Telecommunicatons, 3rd edition (2001)

Download the electronic version of the Encyclopedia of Networking, 2nd edition (1996). It's free!

Contribute to this site

Electronic licensing info



Autonomous System

Related Entries    Web Links    New/Updated Information

Search Linktionary (powered by FreeFind)

Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

An AS (autonomous system) is a collection of networks, or more precisely, the routers joining those networks, that are under the same administrative authority and that share a common routing strategy. An AS has a single "interior" routing protocol and policy. Internal routing information is shared among routers within the AS, but not with systems outside the AS. However, an AS announces the network addresses of its internal networks to other ASes that it is linked to. On the Internet, an AS is an ISP (Internet service provider), but universities, research institutes, and private organizations also have their own ASes.

A group of autonomous systems that share routing information is called a "confederation of ASes" (an interesting phrase when pronounced incorrectly). Autonomous confederations are assumed to have a higher level of trust protection against routing loops among member systems.

Two Internet RFCs discuss autonomous systems: RFC 1930 (Guidelines for creation, selection, and registration of an Autonomous System, March 1996) and RFC 0975 (Autonomous confederations, February 1986)

According to RFC 1930 , "Without exception, an AS must have only one routing policy. Here routing policy refers to how the rest of the Internet makes routing decisions based on information from your AS." Organizations with ASes are assigned 16-bit numbers by IANA (Internet Assigned Numbers Authority). Note: AS numbers are not related to IP addresses.

Figure A-19 illustrates two autonomous systems. IGPs (Interior Gateway Protocols) are used within the AS and EGPs (Exterior Gateway Protocols) are used between the ASes. IGPs provide basic routing within autonomous systems, while EGP is designed to provide reachability information, both about neighbor gateways and about routes to non-neighbor gateways. The IGPs collect the reachability information and the EGP advertises that information.

[Figure 19: See book]

The following exterior gateway protocols operate within autonomous systems to gather internal routing information. See the related entries page for related topics.

The primary exterior gateway protocol used on the Internet is BGP (Border Gateway Protocol). Refer to the BGP topic for more information about autonomous system design and structure.

Refer to "Routing on the Internet" for a historical perspective on the development of autonomous systems and the protocols associated with them. The topic "Routing" provides additional information and links to associated sections.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.