What are an autonomous system (AS) and an ASN?
You’ve probably heard of terms like AS, ASN and BGP. But do you know what each one means? And how do they work together to maintain the internet? In this article, we’ll explain the general concepts of each term, as well as how to become an autonomous system.
The Internet connects networks under different administrations. An autonomous system (AS) is a set of routers under a common external routing policy; an ASN is the number identifying that system. AS and ASN are therefore not synonyms. RFC 1930.
The Border Gateway Protocol (BGP) exchanges network reachability information between autonomous systems according to routing policies.
What’s the relationship with BGP?
BGP advertises paths to IP prefixes and applies policies to select routes. The selected path follows each network's rules; it is not necessarily the shortest path or the one with the lowest latency.
ASs establish interconnections and define how to announce, accept and forward routes. An AS may originate its own prefixes and receive route announcements from customers or partners, depending on its policies.
The importance of autonomous systems and BGP lies in the ability to interconnect different networks, enabling efficient and reliable routing of network traffic across the Internet. Without these elements, communication between different networks would be much more difficult and prone to failure.
How do you obtain an ASN and operate an AS?
In Brazil, organizations can consult Registro.br/NIC.br for ASN allocation requirements; other regions use their respective registries. Receiving a number is only one step: operating an AS also requires routing policy, interconnections and BGP configuration. Registro.br.
How important are ASNs?
An ASN identifies an AS in routing exchanges. Together with BGP and interconnections, it helps separately administered networks communicate across the Internet.
Each AS is responsible for managing traffic routing within its own network, as well as to other autonomous systems. Autonomous systems determine how to forward network traffic and which paths are preferred for different types of traffic through the routing policies they define.
BGP exchanges reachability information between autonomous systems. Policies and interconnections enable forwarding between networks, but the protocol alone does not guarantee optimal performance or freedom from failures.
Operating an AS lets an organization define its own routing and connectivity policies within the limits of its interconnections and agreements with other networks.
In short, ASs and their routing policies are central to interconnecting networks. Operating one requires more than receiving an ASN: interconnections, route policy and ongoing network management also matter.