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Re: selective prepends...one more time

  • From: jlewis
  • Date: Mon Sep 30 19:09:36 2002

On Mon, 30 Sep 2002, Leo Bicknell wrote:

> In a message written on Tue, Oct 01, 2002 at 12:16:07AM +0200, Iljitsch van Beijnum wrote:
> > I think most customers don't know how this works. Maybe someone should
> > write a book that explains this kind of stuff...
> 
> I'm not so sure I'd come to that conclusion.  I think when most
> customers see a problem on their transit providers network they

Some NOCs, even ones that support this on their network, don't understand 
it...or at least have staff that don't even come close to grasping it.  It 
wouldn't surprise me at all if it's beyond a great many customers.

> Most people I see using this feature fall into two catagories.
> The first type doesn't believe the provider can fix the problem
> but is forced to use them (due to price, management, whatever) and
> uses this to avoid their NOC because it doesn't work.  The second
> type of person believes they can actually do a better job of routing
> than their upstream.  This may even be true in some cases (where
> the customer has several transit providers all supporting this
> 'feature'), however I suspect in many cases the customer is actually
> making their own life worse.

Consider a network with several transit providers.  Each transit pipe is
incapable of handling all that network's traffic.  The pipes may even be
of wildly different sizes.  Letting BGP decide where traffic goes (or
comes from) with no tuning just won't work.  You'd end up with some pipes
overutilized and others underutilized.  In this case, selective prepends
make it possible to shift traffic around or decide "we're going to try
forcing all ASxyz traffic to come to us over pipe A."

There's also the occasional case of medium to long term overloaded peering 
between large providers in which case you might want to do all you can to 
force traffic to/from ASxzy to not come/go via pipe A.

> increased.  Even if we assume all the people using it really need
> it, is it worth risking the performance of 500 or 1000 customers
> for the 5-10 who actually use the features?

I wonder if anyone from Sprint or C&W is willing/able to say what 
percentage of multihomed customers utilize these communities?  With enough 
full views from enough route-servers, it might even be possible to analyze 
the data and see how many use this.
 
----------------------------------------------------------------------
 Jon Lewis *jlewis@lewis.org*|  I route
 System Administrator        |  therefore you are
 Atlantic Net                |  
_________ http://www.lewis.org/~jlewis/pgp for PGP public key_________





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