Before, I know the AS path of BGP properties, such as the 200 100i means the route originate from AS100, use network import the route and through AS200, however in my case:

admin@MX480> show route

inet.0: 838836 destinations, 1788129 routes (833714 active, 0 holddown, 863778 hidden)
+ = Active Route, - = Last Active, * = Both   *[BGP/170] 7w2d 09:22:57, localpref 132
                      AS path: 58931 I, validation-state: unverified
                    > to via gr-3/1/10.8
                    [BGP/170] 3w3d 19:51:36, MED 10, localpref 110
                      AS path: 3257 174 I, validation-state: unverified
                    > to via ae5.0
                    [BGP/170] 2w0d 21:02:26, localpref 105
                      AS path: 4809 4134 I, validation-state: unverified
                    > to via ae2.0

you see the route originated from three different AS. whether in the Junos the AS path is not as the same as my learned way?

1 Answer 1


It is possible to originate one prefix from multiple different ASN. This used to be very rare, and I think the primary reason for that is:

  1. some IRR databases, like RADB, didn't support having the same prefix published with two different origin: <ASN>
  2. your announcement would show up in The CIDR Report as suspicious (probably not true anymore)
  3. since others weren't doing it, the idea was untried, and it was a while before the practice started to see practical application (like anycasting itself was technically feasible for a long time, and is now very popular, but at first there was doubt about compatibility for some applications, etc.)

You could email the technical contact for that prefix and ask them.

  • 1
    For the record, this sort of thing has been done since the 90's. Multi-homing being much more common today, it should be trivial to find cases like this. (If I have my own address space, but don't want to run BGP myself, I can have my providers announce the space for me.)
    – Ricky
    Mar 13, 2021 at 16:56
  • Yes, it's a historical concern at this point, no longer a current one. However, more recently than the 1990s, folks were avoiding inconsistent-origin-as deployments due to these concerns. Here's an example tutorial/presentation from Philip Smith at a 2004 NANOG explaining the concern archive.nanog.org/meetings/nanog32/presentations/smith.pdf and you can find similar worries in that era in the proceedings of a few other conferences. Mar 13, 2021 at 17:30
  • Also, I did not remember this until doing a bit more Googling this morning, but several router OSes including Cisco IOS had CLI commands for listing routes with inconsistent origin AS. I am a bit curious what customers requested that feature. Perhaps it may have been useful when doing AS merges, especially in corporate/enterprise networks? Mar 13, 2021 at 17:36
  • Like I said, customers that don't want to run BGP. (and one that wanted our AS first -- but that was an under-the-table move that would not work today.) 'tho I should say, customers that don't know how to run BGP.
    – Ricky
    Mar 13, 2021 at 19:25

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