Autonomous Systems and ASNs

Before configuring Border Gateway Protocol (BGP), it is critical to understand the architecture of the Internet. The Internet is not a single unified network; it is a “network of networks.”

What is an Autonomous System (AS)?

An Autonomous System (AS) is a large network or group of networks that has a unified routing policy. Every AS is controlled by a single administrative entity, such as an Internet Service Provider (ISP), a large enterprise, a university, or a government agency.

Instead of routers knowing every single path to every single IP address in the world, the Internet routes traffic between Autonomous Systems.

The Autonomous System Number (ASN)

To participate in global BGP routing, an organization must be assigned an Autonomous System Number (ASN).

• Public ASNs: Assigned by Regional Internet Registries (RIRs) like ARIN, RIPE, or APNIC. They are globally unique and used on the public Internet.
• Private ASNs: Similar to private IP addresses (e.g., 192.168.x.x), Private ASNs range from 64512 to 65534 (for 16-bit) and are used for internal BGP peering where the ASN is not advertised to the global Internet.

IGP vs EGP

• IGP (Interior Gateway Protocol): Protocols like OSPF or IS-IS are used to route traffic inside your Autonomous System.
• EGP (Exterior Gateway Protocol): BGP is the only EGP in use today. It routes traffic between different Autonomous Systems using their ASNs.

In the next lesson, we will use ASNs to configure our first eBGP peer.