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Prefix length filtering at the ISP-ISP edge

  • From: Alexandre.Gibouin
  • Date: Tue Mar 13 08:43:22 2001 


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
Internet?

- 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
ranges?


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

Thanks for your help
Alex