What is an Autonomous System?

The Internet is a collection of arbitrarily connected Autonomous Systems. An Autonomous System (AS), is classically defined as a set of routers under a single technical administration. Those routers use an interior gateway protocol to route packets within the AS, and then use an exterior gateway protocol to route packets to other Autonomous Systems. Even when multiple interior configurations are constructed, the administration of an AS appears to other ASs to have a single, coherent interior routing plan and presents a consistent picture of what destinations are reachable through it.

AS’s are assumed to be administered by a single administrative entity, at least for the purposes of representation of routing information to systems outside of the AS.

How are Autonomous Systems connected?

Autonomous systems are connected in two ways. The first way is physically, there is a direct connection between two AS’s. The second way is through a BGP connection, which is a protocol that travels over a TCP connection (port 179) between the two AS’s BGP daemons. An AS communicates to another AS over a BGP session. This BGP session communicates routes taht can be reached through the advertising AS.

The primary function of a system that speaks BGP is to exchange network reachability information with other BGP systems.


routing daemons

Learning BGP

One way to figure out BGP would be to setup a routing daemon in a simulated environment. Doing this in the production environment would be ill-advised, but maybe it could be done locally on a pair of home machines that talk to each other over the internet, or perhaps over a shared VPN connection.

Another possibility would be to try marionnet which is a virtual network laboratory and simulation tool. The description reads, “It allows users to define, configure and run complex computer networks without any need for physical setup. Only a single, possibly even non-networked GNU/Linux host machine is required to simulate a whole Ethernet network complete with computers,
routers, hubs, switchs, cables, and more. As Marionnet is meant to be used also by inexperienced people, it features a very intuitive graphical user interface (using GTK2).”