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North American Network Operators Group

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Re: Minimum prefix length?

  • From: bdragon
  • Date: Wed Jun 18 01:52:56 2003

> A few years ago I had an issue with a few of the larger carriers rejecting
> my routes (from a natural Class B space) because their prefix length was too
> short (at one point I simply had the /16 divided into two /17's and this
> still got rejected in some places).  I can't remember which carriers
> exactly, but it may have been some larger transit providers like
> AboveNet/etc.
> Anyone know what the current attitude is by carriers about this?  Nowadays
> with ever-growing memory and CPU it doesn't seem like it's as much of an
> issue.  In an environment where we're all trying to conserve address space
> watching natural boundries doesn't seem all that smart.

It is rare that providers filter on classful boundaries. What is common
is filtering on RIR allocation boundaries. It just happens that in
128/2 nothing longer than /16 has ever been allocated, ttbomk.

Providers should encourage their customers to always originate their
largest aggregate, and _then_ announce (if necessary) any more-specifics
to those they need to and who agree to accept them.

If networks always originated their largest aggregates there wouldn't be
an issue with filtering out long prefixes. The issue is only when a
network announces only the long prefix, and in effect shoots themselves
in the foot by intentionally limitting their own reachability.

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