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

Tue Oct 9 01:19:22 UTC 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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