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Re: Upstreams blocking /24s? (was Re: How Not to Multihome)

  • From: Jon Lewis
  • Date: Mon Oct 08 21:25:33 2007

On Mon, 8 Oct 2007, Justin M. Streiner wrote:

As far as allowing prefixes longer than a /24, that decision was made when the Internet was considerably smaller than it is now, and many networks adopted /24 as the cutoff point. If you make the cutoff point smaller, what is the new point... /26? /32?

Anything longer than /24 is unlikely to propogate far on the internet. You can all check your filters to see. I just checked mine, and neither Level3 nor Time Warner has tried to send me anything longer than /24 in recent history. If they did, it'd show up as hits on a distribute-list deny rule.

Rather than ISPs relaxing filters, you're likely to see them get more strict, filtering shorter prefixes, when routers start falling over in the next few months.

Many networks see customers multi-homing as pretty easy justification to provide them with a /24 of PA space, even if they're small enough that justifying a /24 while single-homed wouldn't work.

This is actually in the ARIN "rules". Multihoming is justification (regardless of utilization) for one of the multihomed network's providers to assign them a /24.

 Jon Lewis                   |  I route
 Senior Network Engineer     |  therefore you are
 Atlantic Net                |
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