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Re: how is cold-potato done?

  • From: Leo Bicknell
  • Date: Wed Jun 26 14:38:21 2002

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
> both cities with an AS-path of X, how do I know which city to use for an
> exit?  I can understand how if X uses communities to tag the geographic
> origin of the traffic, but I'm not aware of many networks that do
> this.  Lots of networks claim to use cold-potato routing though, so how do
> they do it?

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.

-- 
       Leo Bicknell - bicknell@ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/
Read TMBG List - tmbg-list-request@tmbg.org, www.tmbg.org




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