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Re: how is cold-potato done?

  • From: dre
  • Date: Wed Jun 26 15:23:12 2002


Shortest-exit is the default because of the BGP decision process.
This tends to favor heavy-content providers because the bulk of
the data travels shorter distances out of the AS sending content
to the AS receiving the content to their eyeballs.

Shortest-exit is caused by IGP metrics (which shouldn't ever be
the same for two paths, unless you actually want that to happen).
IGP metrics are generally set by length of fiber paths or delay
values.  Provider backbones set these manually with ISIS or OSPF
costs.

There are many ways to do best-exit.  People are always coming
up with strange ways to do routing (ToS routing, MPLS-TE, DS-TE),
and they can sometimes apply these techniques to best-exit.

For those looking for something simple and standard, the two
ways were made known in the first email -> outbound MED's and
delay-based routing from `traceroute' information.  There are
quite a few problems with this as well, documented in many
various papers on the matter e.g.:
http://www.ietf.org/internet-drafts/draft-ietf-idr-route-oscillation-01.txt

For MED's, Avi spoke to the methods used in the following talks:
http://www.nanog.org/mtg-9901/ppt/bgp102/index.htm
http://www.nanog.org/mtg-9811/ppt/avi/index.htm

One thing Avi mentioned here, I never quite understood..
http://www.nanog.org/mtg-9811/ppt/avi/sld031.htm
He says "set MED's in one direction only", but he doesn't say
which direction or why.

As to solving the aggregation problem making outbound MED's
insignficant, there is some work trying to be solved using
Communities (NO-PEER, supercommunities, redistribution, cost
communities, link-bw, et al).  Some of which is believed (and
probably rightly so) to be overcomplicated and possibly even
oscillatory just like the other methods.

I enjoy the simple approach that RFC 3272 takes (surprisingly
simple Inter-Domain traffic engineering coming from the super
complex Intra-Domain TE based on MPLS/etc that the authors
recommend).  They have some suggestions on setting local_pref
and inbound MED's that I found to be very clueful.
http://www.ietf.org/rfc/rfc3272.txt (Section 7.0)

  "Inter-domain TE is inherently more difficult than intra-domain TE
   under the current Internet architecture.  The reasons for this are
   both technical and administrative."

So maybe best practice today for doing best-exit is simply having
the technical data (communities, tags, traffic, etc) and talking
directly with the administrators of your peer-AS to find a solution
(or reading their minds without their data, or inferring it, or
guessing).

I guess the final question is -- why is anyone concerned about
best-exit at all?  Doesn't shortest-exit still get the traffic
there?  I'm willing to bet there are a lot of different answers
to all these questions.

-dre

On Wed, Jun 26, 2002 at 02:35:55PM -0400, Leo Bicknell wrote:
> 
> In a message written on Wed, Jun 26, 2002 at 01:52:08PM -0400, Ralph Doncaster wrote:
> > If I peer with network X in cities A and B, and receive the same route in
> 
> Wow, I'm amazed at the wrong answers here.  The vendors even document
> this, as do the RFC's, see
> http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm
> 
> More to your question, cold-potato uses MEDS to determine the best exit.
> Generally they do not work for large aggregates of the peer, so they
> are spread out across the network.  Clueful peers set the outgoing meds
> on their aggregates to all the same value.
> 
> Set to the same value, or clobbered on inbound, if there is no MED,
> then the routers inside your network will choose the closest exit
> based on your IGP cost.  This is "hot potato" routing.
> 
> If, by strange chance, you have equal IGP costs to two peering points
> with equal MEDS, then it will choose the one with the lower router ID.
> 
> As you can see, there are many other steps to the selection process,
> as documented in the link above.




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