Re: Cox AS22773 uPRF issue - please contact off list

[William Herrin via NANOG <nanog () lists nanog org>]

On Wed, Dec 3, 2025 at 8:32 PM Frank Habicht via NANOG
<nanog () lists nanog org> wrote:
> if you're a customer, using only my PA space, and multihomed:
> I'll do BGP with you -- you can be AS64512.
> I'll do strict uRPF with a fail-filter allowing all my PA space sourced
> by you.
>
> Is there a problem with that?

Most likely, yes there is.

I can drop my announcement without dropping the BGP session. There are
lots of reasons to do so. If you're doing strict URPF, you'll start
blackholeing packets I send to you on the link based on the routes
you're still sending to me, even though they're from the address space
you assigned to me. URPF will show the return route transiting the
other link.

It's even more dicey if the multihoming isn't two links with you but
rather a link with you and another with someone else.

Regards,
Bill Herrin

-- 
For hire. https://bill.herrin.us/resume/
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