[arin-ppml] Rationale for /22

Jon Lewis jlewis at lewis.org
Tue Jul 28 14:09:30 EDT 2009

On Tue, 28 Jul 2009, Scott Leibrand wrote:

> No, we're talking about multihomed organizations here.  If my singly-homed 
> customer gets a /24 from me (out of one of my /16s), then that doesn't add to 
> the table (the only announcement is my /16).  If, however, a multihomed 
> customer gets a /24 from me, they'll announce the /24 as well (both to me and 
> to their other upstream), thereby adding an additional route to the global 
> table (for anyone who doesn't filter /24s, which very few networks do today).
> If the multihomed downstream customer gets their /24 from ARIN instead of 
> from me (their upstream), then it still adds one route to the table.  The 
> only difference is that it can't be filtered without affecting reachability 
> (for example, by someone with hardware that can only do 256k routes).

The distinction some people may not be getting is that if I know ARIN 
allocates from a /8 nothing longer than /20s, then if I'm running out of 
routing slots, I can use a prefix-list to ignore anything /21 (or maybe 
/22) or longer from that /8.  If ARIN allocates /24s from a /8 or probably 
longer net, then I need to accept those /24s.  That's the theory anyway.

Having looked into this some time ago while using Sup2's for BGP, I know 
the unfortunate reality is, even in /8s where there is a RIR published 
minimum allocation size, you'll find clue-deprived networks deaggregating 
their allocations and not announcing the aggregates.  If you filter on RIR 
allocation minimums (even with a bit or two of padding) and don't point 
default at a network that doesn't filter similarly, you're going to have 
reachability issues.

Doesn't this nullify the first point?  i.e. ARIN shouldn't allocate /24s, 
because we want people to be able to filter on RIR allocation minimums 
without losing reachability.  We already know that doesn't work without 
default routing.

What other real world reason is there for not lowering the bar to /24?

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