North American Network Operators Group

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

  • From: Justin M. Streiner
  • Date: Mon Oct 08 21:01:34 2007

On Mon, 8 Oct 2007, David Conrad wrote:

Others have indicated that such filters (assuming they exist) will not last in the face of paying customers presenting longer than /24 prefixes for routing. Specifically, that ISPs will relax their filters (allowing longer than /24) in order to get their peers to accept their long prefixes. Anybody have an opinion on the likelihood of this?

The only exceptions I've seen to the /24 policy are when the customer in question multihomes to the same upstream - sometimes done with a specific AS designated for that purpose, i.e. what UUNET does with AS7046. Those routes are then aggregated that provider's parent block(s).

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? 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.