BGP Private and Public AS Range

BGP Private and Public AS Range

Mohammad Mansouri Mohammad Mansouri

Mohammad Mansouri

发布日期: 2019年2月10日
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Just like IP addresses, ASNs (Autonomous System Numbers) have to be unique on the Internet. The main reason for this is that BGP uses the AS number for its loop prevention mechanism. When BGP learns about a route that has its own AS number in its path then it will be discarded.

Here’s an example:

Above we have three routers, R1 and R3 are using the same AS number. Once R1 sends an update, R2 will accept it but R3 will not since the AS number is the same.

To prevent the above from happening, IANA is in control of the AS numbers (similar to public IP addresses). If you want an AS number for the Internet then you’ll have to request one. They started with 16-bit AS numbers (also called 2-octet AS numbers) that were assigned like this:

  • 0: reserved.
  • 1-64.495: public AS numbers.
  • 64.496 – 64.511 – reserved to use in documentation.
  • 64.512 – 65.534 – private AS numbers.
  • 65.535 – reserved.

Reference: Networklessons.Com

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