Autonomous System (AS)

1. What is an Autonomous System?

An Autonomous System (AS) is a group of IP networks operated by one or more network operator/s which has a single and clearly defined external routing policy. Exterior routing protocols are used to exchange routing information between Autonomous Systems.

2.When should an Autonomous System be created?

An AS needs to be created if a network connects to more than one AS with different routing policies. Some common examples of Autonomous Systems are networks connected to two or more upstream service providers or exchange points, networks peering locally at exchange points.

3.What is an Autonomous System Number?

A public AS has a globally unique number, an Autonomous System number (ASN), associated with it; this number is used in both the exchange of exterior routing information (between neighboring Autonomous Systems), and as an identifier of the AS itself.

There are two types of Autonomous System numbers; Public AS numbers and Private AS numbers.

4.When is a Public Autonomous System number required?

A Public AS number is required only when an AS is exchanging routing information with other Autonomous Systems on the public Internet. That is, all routes originating from an AS is visible on the Internet.

5. Am I eligible for a Public Autonomous System Number?

An organisation is eligible for an ASN assignment if it:

  • is multihomed (this includes organisations connected to a public Internet Exchange Point); and
  • has a single, clearly defined routing policy that is different from its providers’ routing policies.

An organisation will also be eligible if it can demonstrate that it will meet the above criteria upon receiving an ASN (or within a reasonably short time thereafter).

6. When can I use a Private Autonomous System number?

A Private AS number should be used if an AS is only required to communicate via BGP with a single provider. As the routing policy between the AS and the provider will not be visible in the Internet, a Private AS Number can be used for this purpose.

The IANA has reserved AS64512 through to AS65535 to be used as private ASNs.

7. I plan to change my upstream providers – can I take my ASN with me?

This depends on how you received that ASN. If you got it directly from APNIC or an NIR, then it is portable and you can take it with you to whichever providers you choose (subject to the agreement you signed with APNIC or the NIR).

However if got your ASN from an LIR, you can only use it while you continue to receive connectivity from the LIR. In other words, if you decide to no longer use that LIR as one of your upstream providers, then you will have to return the ASN.

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