Re: how about a well-known DOWNGRADE BGP Community? (Was: RTBH Support Across the Industry)

[Saku Ytti via NANOG <nanog () lists nanog org>]

You won't need the MPLS family to map DCU to class, because at the
time when we do the lookup, it's either IPv4 or IPv6. If we do mpls
lookup, we've already decided that it's BE or LE.

This is the ingress PE which will decide how the packet goes through
the network and the transit P doing MPLS lookup will just honor the
EXP bits as imposed by ingress PE.

Of course the same config works fine without MPLS too, and the transit
node honors the DSCP bits in absence of EXP bits.

On Tue, 4 Aug 2026 at 20:06, Michael Hare <michael.hare () wisc edu> wrote:
> Saku-
>
> Thanks for link.  This is what I had in mind, but I presumed if output interface was into an LSP maybe I'd need 
> output filter in family MPLS.  At this point, I'll just try this out in the lab.
>
> -Michael
>
> > -----Original Message-----
> > From: Saku Ytti <saku () ytti fi>
> > Sent: Tuesday, August 4, 2026 11:30 AM
> > To: North American Network Operators Group <nanog () lists nanog org>
> > Cc: Michael Hare <michael.hare () wisc edu>
> > Subject: Re: how about a well-known DOWNGRADE BGP Community? (Was:
> > RTBH Support Across the Industry)
> >
> > On Tue, 4 Aug 2026 at 18:20, Michael Hare via NANOG
> > <nanog () lists nanog org> wrote:
> >
> > > +1.  Was going to follow up to Saku's message about how/if folks are doing
> > that in JunOS [or other OS] natively without outside automation.   Our backbone
> > has scavenger style plp in BE, I control admission via prefix-list in ingress fwfilter
> > on untrusted interfaces.  Haven't personally tested but looks like DCU +
> > forwarding-table export policy + output firewall filter, maybe this is possible
> > today but requires output fwfilter to family mpls.  I don't currently have a
> > fwfilter there so ideally another option exists.
> >
> > Something to this note, nothing really specific to any AFI:
> > https://urldefense.com/v3/__https://raw.githubusercontent.com/job/draft-
> > downgrade-bgp-
> > community/refs/heads/main/nos/junos.txt__;!!Mak6IKo!IOVZQT4Lf7E7zL5MWS
> > bICpNXJnyzuaVs2NnLxqdQJftUH3NI3k00Ws0VNddwQDb250bo4F95RdUFgnqM$
> >
> > For context, remainder of QoS config would look something to:
> > https://urldefense.com/v3/__https://p.ip.fi/Nott.txt__;!!Mak6IKo!IOVZQT4Lf7E7
> > zL5MWSbICpNXJnyzuaVs2NnLxqdQJftUH3NI3k00Ws0VNddwQDb250bo4F95Rb4
> > m1HQV$
> >
> >
> >
> > --
> >   ++ytti



-- 
  ++ytti
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