Smaller than a /24 for BGP?

Robert McKay robert at
Wed Jan 25 02:55:44 UTC 2023

On 2023-01-25 00:10, Chris J. Ruschmann wrote:
> How do you plan on getting rid of all the filters that don’t accept
> anything less than a /24?
> In all seriousness If I have these, I’d imagine everyone else does
> too.

Allow someone to advertise a covering /24 (and route onward to the 
longer prefixes) in exchange for being able to 'research' the traffic?

You do want a cover advertisement, but it will hardly ever be used by 
anyone. It attracts traffic, but before that traffic gets anywhere near 
the origin it hits the more specific longer prefixes and goes straight 
there.. probably via your immediate upstream, which it would have 

There are large sections of space that have historically always had 
cover advertisements - for instance. If cogent started 
accepting /29's in there they'd work perfectly courtesy of 38/8 - with 
nobody else making any other changes to anything.

Probably before long the other transit ISPs would start accepting the 
longer prefixes too and you'd have fairly functional multi-homing. The 
long tail of other ISPs doesn't even matter since they inevitably hand 
the traffic over to someone who will know what to do with it.


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