Autonomous system

In computer networking, autonomous systems (also domains) are regions of aggregated groups of routers. They’re an important construct in the network layer.

Basics

Some basic ideas:

• Each autonomous system consists of multiple networks under a single administrative authority.
  • “multiple networks” — physical networks like LANs, etc; IP subnets; and different routing domains within the organisation, like those that might belong to different departments (administration, student housing, etc.)
  • “administrative authority” — in Canada, these would be big telcos like Rogers, Bell, or Telus.
• Each autonomous system should advertise its networks to other AS by sending information to one of the routers in the core AS. Routers must carry the AS number.

We define a few types of AS:

• A stub AS only has one connection to another AS. Data traffic can either be initiated or terminated in a stub AS (but not flow through!).
• A multihomed AS can have more than one connection to other autonomous systems. They should not route the traffic of the AS to which they are connected.
  • i.e., they function like a stub AS in that they initiate or terminate data. They just have more links to outside AS that they can send data across.
• And a transient AS is connected to more than one AS and allows traffic to pass through.

There are two types of routing.

• Routing intra-AS is between routers within the same AS. All routers in the AS must run the same intra-domain protocol. Routers in different AS can run different intra-domain routing protocols. At the “edge” of the AS, a gateway router has a link to routers in other AS.
  • The most common intra-AS routing protocols are: RIP, EIGRP, and OSPF.
  • Single admin, so policy (control over how its traffic is routed or who routes through the network) is not a priority.
• Routing inter-AS is among AS. Gateways perform inter-domain routing (as well as intra-domain routing).
  • A datagram that is destined for an outside AS must be forwarded to a gateway router that might have a connection.
  • Inter-domain routing must learn which destinations are reachable through directly adjacent AS, then propagate this reachability information to all routers in the self AS.
  • The primary inter-AS routing protocol in use is BGP.

Within the gateway router, there’s a forwarding table that’s configured by both the intra- and inter-AS routing algorithms.