Skip to main content

Autonomous System (AS)

An Autonomous System, commonly abbreviated as AS, is a collection of IP routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the internet. Essentially, an AS is a group of IP networks operated by one or more network operators that have a single and clearly defined external routing policy.

Characteristics of an Autonomous System​

  1. Control and Policy: An AS is controlled by one or more network operators who oversee its internal routing policies. They also manage the AS's external routing policies, which dictate how the AS exchanges routing information with other ASes.

  2. Unique Identifier: Each AS is assigned a unique AS number (ASN) which is used to identify it on the internet. ASNs are crucial for the operation of BGP.

  3. BGP Usage: ASes use BGP to exchange routing information with other Autonomous Systems. BGP is the protocol that makes the internet work, allowing different ASes to communicate with each other.

  4. Routing Autonomy: Within an AS, the network operator can use any internal routing protocol (like OSPF, EIGRP, or RIP) to route packets. However, for routing between ASes, BGP is used.

Use Cases and Importance​

  1. Internet Routing: ASes are the building blocks of the internet's routing architecture. The internet is essentially a network of networks, each network being an AS.

  2. Policy Implementation: Different ASes can implement different routing policies based on their requirements, such as traffic engineering, load balancing, and route preference.

  3. Organizational Boundaries: An AS often aligns with organizational boundaries. For example, a large corporation, a university, or an ISP each might operate its own AS.

  4. Multi-Homing and Redundancy: Organizations use ASes for multi-homing, which involves connecting to multiple ISPs for redundancy and improved network performance.

  5. Global Connectivity: ASes enable global connectivity by allowing different networks around the world to communicate and exchange data.

Conclusion​

An Autonomous System is a critical concept in networking, especially concerning the global routing architecture of the internet. By having a unique ASN and the ability to run BGP, ASes facilitate the exchange of routing information across the internet, allowing disparate networks to interconnect and communicate effectively. Understanding ASes and their role is essential for network engineers involved in larger-scale or ISP-level networking.