[arin-ppml] Rationale for /22

William Herrin bill at herrin.us
Tue Jul 28 11:56:00 EDT 2009

On Tue, Jul 28, 2009 at 11:19 AM, Kevin Kargel<kkargel at polartel.com> wrote:
> I haven't been following this closely, sometimes work intrudes, but have we
> all forgotten all the rationals that say that unaggregated /24's break
> things?  Or are we assuming that everyone will be able to handle huge
> routing tables or are we assuming that the /24 recipients don't care if they
> can route beyond peers?

Hi Kevin,

I thought the problem was a large number of disaggregate announcements
from an org's single network topology needlessly fill expensive TCAMs.
I'm not aware that the size of the announcements has anything to do
with it; just the count. Right?

Anyway, we're talking about multihomers here, so I don't think we're
talking about adding new route announcements at all, are we? We're
talking about folks who are announcing a cut-out /24 from an ISP's
larger block (NRPM and allowing them to announce a direct
ARIN assignment instead. Same address count. Same consumption in the
routing table. But without all of the nasty problems associated with
cutting a /24 out of an ISP's block.

Some of the problems with cutting a /24 out of an ISP's block include:

* Causes major grief with reverse path filtering / spoofing
protection. If the ISP implements this at all, multihomed cutouts have
to be explicitly excluded. In effect, ties the ISP's hands with
respect to his internal network management.

* There are some funky issues with BGP which make withdrawing a cutout
route a lot slower than re-establishing an independent route.

* Grief with SWIP reporting errors that cause administrative hassles
getting the addresses announced properly.

* RIR boundary filtering that causes multihoming to malfunction during
major network partition events (when you most want multihoming to work

* Renumbering when you change vendors.


William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004

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