DMARC ViolationAS21299 - ASN prepending 255 times

Matthew Petach mpetach at
Sat Mar 26 00:17:02 UTC 2022

On Fri, Mar 25, 2022 at 2:59 PM Adam Thompson <athompson at>

> Tom, how exactly does someone “ride the 0/0” train in the DFZ?

It's not so much "ride the 0/0 train" as much as it is
"treat excessive prepends as network-unreachable"

Think of prepends beyond say 10 prepends as a way
to signal "infinite" distance--essentially, "unreachable"
for that prefix along that path.

Anyone that is prepending to do traffic engineering is
doing *differential* prepending; that is, a longer number
of prepends along one path, with a shorter set of prepends
along a different path.

So, dropping the inbound announcement with 255 prepends
merely means your router will look for the advertisement with
a shorter number of prepends on it.

If you're only announcing one path for your prefix, and it is
prepended 255 times, you're fundamentally not understanding
how BGP works, and the only way to get a clue-by-four might
be to discover you've made your prefix invisible to a significant
portion of the internet.

> I’m connected to both commercial internet and NREN, and unfortunately-long
> paths are not uncommon in this scenario, in order to do traffic steering.
> If there’s another solution that affects global *inbound* traffic
> distributions, I’d love to hear about it (and so would a lot of my peers in
> edu).
> If there were a usable way to “dump” the excessively-long path only as
> long as a better path was already known by at least one edge router, that
> might be workable, but you’d have to keep track of it somewhere to
> reinstall it if the primary route went away… at which point you may as well
> have not dropped it in the first place.
You dump the excessively-long path based on the assumption that
the only reason for a long set of prepends out one path is to shift traffic
away from that path to one that you're advertising out with a *shorter*
set of prepends.

The router doesn't need to 'look' for or 'keep track' of the different
path; the human makes the decision that any sane BGP speaker
would only prepend 255 times on a path if there was a shorter
as-path advertisement they wanted people to use instead.

So, drop the excessively long prepended path, and make use
of the 'should be in the table somewhere' advertisement of the
prefix with fewer prepends.


> -Adam
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