From: Danny McPherson <[email-masked]>
Date: Thu, 06 Jun 2002 14:38:53 -0600

> One possible solution is to mandate 'always-compare-med' behaviour
> which will make paths have a global ordering. I remember there was
> strong reaction against this from the operational folks but i'm not
> quite sure what the arguments where (would apreciate if somebody could
> refresh my memory).

Correct me if I'm wrong -- but the primary use of MEDs is 
for a BGP speaker to convey the optimal entry point for
a given destination to a peer AS.  That is, for one AS to 
associate MEDs with given reachabilty based on some policy 
internally consistent within the AS.  Typically, the MED
values are derived from IGP metrics associated with BGP 
NEXT_HOPs, thereby reflecting internal route selection 
preference in the process.

Now, beyond that fact that MEDs are largely broken in 
general in the Internet, mostly as a result of 
aggregation...

The issues with comparing MEDs between different ASes is 
that MED values are derived from some policy defined and 
contained internal to the peer network.  

Used to be, comparing MEDs between a network that ran 
IS-IS and a network that ran OSPF would result in the 
network running IS-IS receiving a much larger amount of 
traffic, simply because the IS-IS metric space was 
limited to 6 bits, and the resulting IGP metric-derived
BGP MED from the AS were lower values.

Though the IS-IS metric space isn't so much of an issue 
today, basing BGP path selection on MED values received 
from different ASes, probabilistically (sp?) derived from 
potentially significantly different policies, is certainly 
a BAD idea.

In marketing speak, MEDs intend to turn hot potato routing 
into cold potato routing but end up with mashed potato 
routing. 

-danny