Configuring Internet Protocol (IP) routing

Autonomous systems -- exterior vs. interior routers

As shown in ``Routing table description'', the kernel routing table usually includes entries for a few networks and also includes a default entry. Any packets addressed to networks not found in the kernel routing table are sent to the gateway machine listed in the default entry. This provides an important efficiency in that every machine need not list in its kernel routing table every network its users may need to reach. Instead, a machine need only know about those networks nearby. The default router can be expected to know about networks beyond this group.

On the Internet, this concept is standardized by the registration of autonomous systems (AS). A group of networks on the Internet under a common administration is considered an autonomous system and must be registered as such with the InterNIC Registration Services. Registration will provide an autonomous system number to your autonomous system. You can reach the InterNIC Registration Services through any of the following:

Network Solutions
Attn: InterNIC Registration Services
505 Huntmar Park Drive
Herndon, VA 22070

(703) 742-4777

anonymous ftp to (

For example, all of the networks that form the internetwork of a single business could constitute an autonomous system. In such a situation, a single machine is the gateway between the autonomous system and any networks outside the autonomous system. This gateway to the outside of the autonomous system is called an ``exterior router'' because it is expected to communicate with routers on networks outside of the autonomous system. Any additional routers within the autonomous system are called ``interior routers'' and are expected to communicate only with routers on networks within the autonomous system.

This distinction between interior and exterior routers is important in the selection of routing protocols to be run to maintain the kernel routing tables on your systems. To improve the efficiency of the exchange of network information between routing daemons, some protocols are designed specifically for exchanging interior routing information and some are designed specifically for exchanging exterior routing information.

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© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003