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

["Justin M. Streiner" <streiner () cluebyfour org>]

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.

jms