Prefix length filtering at the ISP-ISP edge

Nothing new in this mail but I need some clarification ...

- To my understanding, the common practice is, at least, to accept any address
block larger or equal to /19 and to filter any block smaller than /24 (except
eventually at the customer-ISP edge). ARIN and RIPE have changed for a while
their minimum allocation size to /20 so I guess there is no risk of routing
blackhole with /20 blocks, right?

- What about PI-blocks between 21 and 24 bits? If a end-user network annouces
such an address block, what is the risk of routing blackholes throughout the

- Each ISP seems to apply its own filtering policy. Nevertheless, what are the
most common filtering practices for the ISP-ISP edge?

- Is it common to have different filtering policies depending on the address

I will be grateful for any link to official or non-official documentation.

Thanks for your help