any dangers of filtering every /24 on full internet table to preserve FIB space ?

William Herrin bill at
Tue Oct 11 20:59:13 UTC 2022

On Tue, Oct 11, 2022 at 1:15 PM Matthew Petach <mpetach at> wrote:
> Wouldn't that same argument mean that every ISP that isn't honoring
> my /26 announcement, but is instead following the covering /24, or /20,
> or whatever sized prefix is equally in the wrong?
> What makes /24 boundaries magically "OK" to filter on,

Hi Matthew,

/24 is the consensus filtering level for Internet-wide routes and it
has been for decades. It became the consensus as a holdover from
"class C" and remains the consensus because too many people would have
to cooperate to change it. Indeed, a little over a decade ago some
folks tried to change it to /19 and then /20 for prefixes outside "the
swamp" and, well, they failed. Likewise, more than a few folks
announce /26's to their immediate transit providers and they simply
don't move very deep into the system -- nobody has any expectation
that they will.

> To wrap up--I disagree with your assertion because it depends entirely
> on a 'magic' /24 boundary that makes it OK to filter more specifics smaller
> than it, but not OK to filter larger than that and depend instead on covering
> prefixes, without actually being based on anything concrete in BGP or
> published standards.

Got any better reasons besides disliking the consensus?

Bill Herrin

For hire.

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