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Re: how is cold-potato done?

  • From: Mathew Richardson
  • Date: Wed Jun 26 18:14:38 2002

> Leo Bicknell <bicknell@ufp.org> [Wed, Jun 26, 2002 at 02:35:55PM -0400]:
>
>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.

<snip>

In the interest of accuracy, it's worth noting that some vendors will
choose the one with the lower router ID, and others will choose
the route that was learned first (at least by default), despite
documentation to the contrary.

mrr




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