How can a prefix have more than one ASN?

Put basically, a prefix (IP range) can be announced by more than one AS at a time because there is nothing in BGP that disallows it to.

This shows up because bgp.tools has many different views of routing tables, and in some routing tables the originating asn for a prefix will be different.

This is commonly due to:

  • Migrations in place where a prefix is announced from two ASNs at once during a network migration/merger
  • Some networks like to run their anycast networks with each anycast “island” as a different ASN. This practice has been documented in RFC6382. (This is rare overall)
  • Some networks may be exporting bad data to bgp.tools or bgp.tools’es data sources, either exposing their BGP Optiomiser routes, or their internal traffic engineering hijacks.

bgp.tools uses the most commonly observed origination ASN for the primary display.

Last updated: 9th October 2021