BGP - Autonomous System (AS)

An Autonomous System (AS) in the context of BGP refers to a collection of IPv4 or IPv6 networks and routers under the control of a single organization that presents a common routing policy to the internet. Each AS is identified by a unique AS number (ASN), which is used for BGP routing.

Here are the key aspects of an Autonomous System in the context of BGP:

  1. Independent Routing Policies: An AS represents a group of IP networks that adhere to a specific set of routing policies set by the organization controlling it. These policies determine how data is routed within the AS and how it interacts with other ASes.

  2. AS Number (ASN): Every Autonomous System is assigned a unique ASN, which is used to identify the AS in the global internet routing table. The ASN is a vital component in the BGP routing process, as it helps in maintaining a clear path of routing data through various networks.

  3. BGP for Routing: BGP is the protocol used by ASes to exchange routing information with each other. It enables ASes to determine the best paths for data to travel across the interconnected networks of the internet.

  4. Path Vector Protocol: BGP is a path vector protocol, meaning it keeps track of the full path of ASes that data packets travel through. This helps in avoiding routing loops and makes the routing process more efficient.

  5. Peering Relationships: ASes form peering relationships with each other to exchange routing information. These relationships can be either transit (where one AS provides connectivity to the rest of the internet for another) or peering (where two ASes exchange traffic between their own customers).

  6. Scalability and Control: The use of ASes in BGP allows for scalable internet growth and provides organizations with control over their own routing policies, which can be optimized for performance, cost, or other organizational needs.

An Autonomous System in BGP is a fundamental building block of internet infrastructure, enabling the organization of the internet into manageable and independently operated segments with their own unique routing policies.