What is as?

AS, or Autonomous System, is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators on behalf of a single administrative entity or organization, that presents a common, clearly defined routing policy to the Internet.

Here's a breakdown of key aspects:

• Definition: An Autonomous System is essentially a network, or a group of networks, that has a unified routing policy.

• Routing Policy: This refers to how an AS advertises and exchanges routing information with other ASes. This is crucial for determining the best path for data to travel across the internet.

• Autonomous System Number (ASN): Each AS is assigned a unique ASN by a Regional Internet Registry (RIR). This number is vital for identifying the AS during routing processes.

• Interior Gateway Protocols (IGPs): Within an AS, internal routing is typically handled by IGPs such as OSPF or IS-IS.

• Exterior Gateway Protocols (EGPs): To communicate routing information between different ASes, an EGP is used. The primary EGP used today is the Border Gateway Protocol (BGP).

• Border Gateway Protocol (BGP): BGP is the de facto inter-AS routing protocol, used to exchange network reachability information among ASes. It is responsible for making routing decisions based on paths, network policies, and cost.

• Peering: ASes often establish peering agreements with each other to directly exchange traffic. This can improve performance and reduce latency.