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   Are there some ISPs who filter prefixes longer than /19 or a /20?. I
thought they filtered only prefixes which are longer than /24?


If the space is from what was originally class a or b, there are some that
will filter based on that. An ex employer of mine did that.


Most notably verio does this.


Yes we do (Swisscom IP-Plus AS3303). We filter on RIR minimal allocations
and anything longer than /21 within historical A/B space