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Autonomous System AS

In the arcane world of exterior routing and default-less routers, Autonomous Systems (AS's) reign as the world's largest routing entities. AlterNet, CERFnet, SPRINTlink - all are Autonomous Systems. See RFC 1772 for a decent discussion of AS's and BGP's use to route among them.

In technical terms, an AS number is a 16-bit integer assigned by InterNIC and used by BGP to implement policy routing and avoid top-level routing loops. A more enlightening view is of an Autonomous System as a collection of CIDR IP address prefixes under common technical management. For example, the CIDR block from 208.130.28.0 to 208.130.31.255, as well as network 201.64.75.0 might be AS 2934. Let's say a host in another network tried to connect to 208.130.28.2. Most likely a series a default routes would deliver the traffic to an ISP router with a defaultless routing table. This router, and others in the Internet backbone, would then route the traffic according to its rules for AS 2934. Once the traffic reaches AS 2934, an interior routing protocol (such as OSPF) would be used to deliver the packets to their final destination.

If you draw a network map of AS's, three distinct types can be identified:

• A Stub AS is only connected to one other AS. For routing purposes, it could be regarded as a simple extension of the other AS. In fact, most networks with a single Internet connection don't have a unique AS number assigned, and their network addresses are treated as part of the parent AS.

• A Transit AS has connections to more than one other AS and allows itself to be used as a conduit for traffic (transit traffic) between other AS's. Most large Internet Service Providers are transit AS's.

• A Multihomed AS has connections to more than one other AS, but does not allow transit traffic to pass, though its interior hosts may route traffic through multiple AS's. This is the typical configuration for a large corporate network with multiple redundant Internet connections, but which does not wish to pass traffic for others.

Interior routing policies and protocols must be established within each AS, enabling it to route packets internally. A Stub AS can usually have a default route to its parent. A Transit AS must use default-less routers. A Multihomed AS may use either default-less routing, or setup a default route to one of its neighbor AS's, but this will probably result in poor quality routing. Exterior routing, with a routing protocol such as BGP, is used to interconnect the various AS's, with their independant interior routing protocols, into a single coherent Internet.