[arin-ppml] FW: Rationale for /22

Chris Grundemann cgrundemann at gmail.com
Wed Jul 29 14:36:29 EDT 2009

On Wed, Jul 29, 2009 at 12:21, Kevin Kargel<kkargel at polartel.com> wrote:
>> I think it's more about what I see of your customer's routes.  If you
>> announce a /20 and your customer via another provider announces a /23 I
>> will always prefer that path.  So, my bits (and your revenue) will go to
>> the other provider.
>> Mike
> Which is absolutely fine by me if that's the way they want to balance their
> traffic.  My revenue is the same whether his circuit is 1% or 99% utilized.
> I don't understand why you would have a problem with it?

But that is not necessarily the way that "they want to balance their
traffic."  You are taking that choice away from them.

Hopefully this is not going too far off topic but just for clarity,
Bill and Mike are correct:

The issue is not about you having or not having the route, the issue
is with other providers having or not having the *best* route.

Let's say Multihomer X is a customer of both ISP A and ISP B.

ISP A assigns 10.0.10/24 to Multihomer X.
Multihomer X advertises 10.0.10/24 to ISP A and ISP B.
ISP A aggregates their space in full and only advertises 10/8 to it's peers.
ISP B cannot aggregate ISP A's space and thus advertises 10.0.10/24 to
it's peers.

PEER Y, who peers with ISP A and ISP B will select it's route to first based on longest match.  So 10.0.10/24 is selected
with no consideration given to 10/8 regardless of Multihomer X'
preference (maybe they are sending ISP A a community to set the local
pref or they are prepending the AS path to ISP B) whatsoever.  No
matter what Multihomer X' expectation or desire, all traffic will flow
to them through ISP B.

PEER W peers with only ISP A and ISP C.  PEER W gets 10/8 from ISP A
and 10.0.10/24 from ISP C (who peers with ISP B).  PEER W has the same
"problem" as PEER Y when forwarding traffic to Multihomer X; the
longest match is 10.0.10/24 and so traffic from PEER W travels through
ISP C, then ISP B and then to Multihomer X - instead of direct through

ISP A not advertising the /24 has determined this behavior, Multihomer
X, ISPs B & C, PEERs Y & W all have no say in it.

> If any of my upstream providers dictated that my traffic had to go through
> them things would get exciting real quick, or I would have a different
> provider.  This would be especially true in the case of a burstable circuit.

Advertising the space that you provide to a multihomed customer would
not force them to go through your network, it would allow them to...

> I have never had an upstream provider complain because my circuit was
> under-utilized.

I guess that depends on how they bill.


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Chris Grundemann

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