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Re: how is cold-potato done?

  • From: Clayton Fiske
  • Date: Wed Jun 26 14:10:23 2002

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?

If they are really doing cold-potato routing, they are listening to
the BGP MEDs (metrics) sent by their peer(s) and making the routing
decision based on that. If the MEDs are the same for both routes, the
IGP metric for each BGP next-hop is likely making the decision.

http://www.nanog.org/mtg-9811/ppt/avi/tsld010.htm

Those are the criteria, in order, which BGP uses to make its decision.
I am assuming synchronization, route to next hop, and router-local
decisions (IBGP vs EBGP, weight) are non-issues in this scenario.
Since localpref would be set internally, and AS path is the same (as
I would assume origin code is), that leaves the MED as the first
criterion, followed by shortest next-hop metric (IGP metric, typically).

-c





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