[arin-ppml] Rationale for /22

William Herrin bill at herrin.us
Tue Jul 28 21:22:35 EDT 2009

On Tue, Jul 28, 2009 at 8:39 PM, Jon Lewis<jlewis at lewis.org> wrote:
> On Tue, 28 Jul 2009, William Herrin wrote:
>> That theory was destroyed by NRPM
> https://www.arin.net/policy/nrpm.html#four236
> How so?  Suppose you're multihomed to Sprint and XO and are using a /24 from
> XO space.  Lets say I'm on two other networks and I filter on RIR minimums.
>  I normally get to you via an XO aggregate route (say a /17 that covers your
> /24).
> Your XO circuit goes down.  I still send traffic destined for your /24 via
> the path I see for the /17.  A few things can happen.  The traffic makes it
> as far as XO, at which point they hopefully are accepting your /24 via your
> advertisement to Sprint, and the traffic gets to you (it's a longer path
> than usual...but at least it made it).  I send traffic for your /24 via the
> path I see for the /17, and before it gets as far as XO, a router in another
> AS along the way has "full routes" and has a path for your /24 via Sprint
> and the traffic gets to you about as efficiently as normal.
> What broke?


In that scenario, nothing but some inefficient routing. Now let's
suppose you're connected to Sprint and Sprint and XO have a peering
spat. You should still be able to get to me since we're both connected
to Sprint but you can't because you filter my /24 route and XO's
covering route has vanished from your view.

Sprint and Cogent had that problem last year. It lasted a good part of a week.

Yes, as an origin AS you can generally filter whatever you want and
spit a default route at one of your ISPs. You can do that regardless
of the RIR minimums. At the DNC I kept a 6500/Sup2 alive an extra
couple of years by filtering APNIC /8's and tying them to the best
path for Japan's routes. Cut almost 60k prefixes from my TCAM.

As a transit AS you really can't get away with that. If you don't
carry all the routes down to /24, your customers will see routing

By the way, do you know of anyone who is actually filtering on RIR
minimums instead of on /24? That would come in handy in one of my
negotiations with a vendor right now.


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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