Autonomous Systems (AS, Networking)

"An AS is a group of IP networks operated by one or more network operator(s) that has a single and clearly defined external routing policy" - APNIC Website

This is the idea that the internet is made of autonomous systems, or a whole cloud (network) such that everything and everyone is controlled per autonomous system.

The reason we'd want this is:

• Scalability
• Can apply policy to our routes

It could be for one or more companies/organizations but to the outside world it looks like one collection of networks (routing prefixes).

Each AS is assigned a 32-bit number for identification:

• Older AS numbers were 16 bits
• Assign by: Internet Assigned Numbers Authority (IANA)

Peering

Your ISP (Internet Service Provider) will provide internet service to customers. Considering the connections/relationships between AS's is called peering.

Types of AS's

Here we briefly describe the types of AS's. Namely there is:

• Stub AS
  • Single connection to the outside world, does not carry anyone else's traffic.
  • Only routes local traffic (either originated within the AS or destined for a network within the AS)
• Mutihomed AS
  • Multiple Connections to the outside world
  • Used for redundancy
  • Will NOT carry traffic for others (ie: is not a Transit AS)
• Transit AS - carries internet traffic
  • An ISP (Internet Service Provider)
  • Will carry transit traffice (traffic is just passing through)
  • An ISP provides connectivity to the customer

Notice that Cal Poly doesn't have any downstream ASs, but we are peers, implying we are a stub AS.

However, the CSU Chancellor's office AS does have outgoing traffic (to Cal Poly), so then they are (likely) a multi-modal AS (since they are not an ISP themselves).

And Alabama Supercomputer Authority has so many peers that they (likely) are a transit AS (although they could definitely be multi-modal).