Policy Routing

Sun Aug 26 05:50:32 UTC 2001

Jon,

Unless I am misunderstand how BGP (and routing at all)
works something very seriously wrong with your suggestion.

Your border router will advertise to a customer only best
and active routes. You cannot advertise to BGP peer two routes
to the same destination. So modifying AS_PATH will not work,
because you will send to Customer X only best selected route
from your border router.

The only way to advertise suboptimal route would be to have
it selected as 'best' on your border router.

Of course, technically, you can advertise one thing
(AS_PATH via NSP A) , and forward it to a different
destination (NSP el'cheapo), but this is another story.

Please correct me if I am wrong -- it is 1:40 am
and I had a long day...

Thanks,

Przemek

-----Original Message-----
From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu]On Behalf Of
jlewis at lewis.org
Sent: Sunday, August 26, 2001 1:27 AM
To: John Fraizer
Cc: Jeff Cates; nanog at merit.edu
Subject: Re: Policy Routing

On Sun, 26 Aug 2001, John Fraizer wrote:

> Rereading your post, one would believe that since "Company X" is a BGP
> customer of yours, you're going to be sending them a full view.  Unless
> there is a knob that I'm not familiar with, that means that you're going
> to be sending them the _BEST_ routes that you see in your core and not
> just those from "NSP A" to which you are proposing to policy-route all of
> "Customer X's" traffic.  If this is indeed the case, I would think that
> policy-routing the customers traffic destined for "prefix Y" via a
> path other than the path listed in the NLRI you're sending "Customer X" on
> their BGP feed is outright fraud.

Suppose you apply an output route-map on their BGP session that prepends
the heck out of paths where peer-A isn't your next as-hop?  That way,
you're giving them full routes and making it clear via BGP that if they
have another path via another provider for non peer-A routes, the other
will be better.

--
----------------------------------------------------------------------
 Jon Lewis *jlewis at lewis.org*|  I route
 System Administrator        |  therefore you are
 Atlantic Net                |
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