[arin-ppml] Rationale for /22

Jon Lewis jlewis at lewis.org
Wed Jul 29 00:48:13 EDT 2009

On Tue, 28 Jul 2009, William Herrin wrote:

> 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.

This was originally going to be my reply to Owen, but then I noticed his 
reply was not to the list.

Ok, there are a few assumptions in the scenario I suggested.

1) "Tier 1's" "don't filter"...i.e. not for route table reduction.

2) Any network that does filter for the purpose of route reduction carries 
a default route pointing towards a network that doesn't doesn't filter or 
that points default at one that doesn't.  This could go on for several 

3) For the sake of this argument, peering battles are ignored.  Yes, they 
happen from time to time, and they're one of the reasons networks 
multihome, but they're hardly the normal state.

Sure, multiple failures (or one failure and a peering game of chicken) can 
happen that would break end to end connectivity, but that's already a 
possibility without filtering.  You can only do so much to guarantee the 
network / internet generally works.

> 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
> anomalies.

I got away with it.  I suspect lots of other transit AS's have.

> 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.

No.  I wrote some emails and a blog entry about this in late 2007 / early 


We ran with a filter based on parts of this for a short time before 
upgrading.  Based on the number of emails I got about it, I suspect others 
used it, but I don't know who or if they still are.

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