DMARC ViolationAS21299 - ASN prepending 255 times

Jon Lewis jlewis at
Sun Mar 27 16:31:11 UTC 2022

On Fri, 25 Mar 2022, Baldur Norddahl wrote:

> On Fri, 25 Mar 2022 at 17:32, Joe Provo <nanog-post at> wrote:
>       That said, prepending pretty much anything more than your current view
>       of the Internet's diameter in ASNs is useless in practice.
> That is one way of viewing it. But prepending can also be used for traffic engineering. I could prepend 1
> to my free peers, 2 to my paid peers, 3 to cheap ip transit, 4 to expensive ip transit etc. The linked
> draft RFC does not appear to discuss this at all. The depth of prepending used this way only relates to how
> many different classes of peers you can imagine in your setup and is not at all related to the "internet's
> diameter".

Is prepending used for any purpose other than TE?  The point I think Joe 
was trying to make was prepending once or even a few times has uses. 
Prepending more than a few times is unlikely to accomplish anything a few 
prepends didn't get done.

Prepending 50, 100, 200+ times is kind of a universal "We have no clue 
what we're doing and you should reject our routes."

Once upon a time, such long prepends would break certain BGP 
implementations, causing session resets when a route like this was 
encountered.  Hopefully, that's not a problem anymore, but enough networks 
likely still block excessive prepends that you shouldn't expect to be able 
to do this and have your route globally accepted...just like you can't 
advertise a v4 /25 and expect global reachability if there are no covering 
aggregate advertisements.

The interesting question here is, "did they really think a few more 
prepends would get the job done?" or did they misunderstand their router's 
prepend function, prepend 21299 (thinking they were telling it to prepend 
their ASN), and that got truncated because the syntax was actually telling 
it how many times to prepend the local AS?  I'm guessing the latter, as 
they seem to have 254 prepends, and I'm guessing 255 is the max number of 
instances of their ASN their router is willing to put on an advertised 

  Jon Lewis, MCP :)           |  I route
  StackPath, Sr. Neteng       |  therefore you are
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