Subject: Re: MED is evil? (was: Re: I-D ACTION:draft-mcpherson-rfc3065bis-00.txt)

From: Danny McPherson <[email-masked]>
Date: Thu, 17 Oct 2002 10:40:43 -0600

> Whether to strip MED depends on what you are trying to accomplish with
> MED.

Of course.  It's just that in my experience the harm caused by MEDs 
considerably outweighs the good.

> Consider the case where ISP A and ISP B peer at multiple places and A
> is directly connected to destination X and B is directly connected to
> destination Y.
> 
> Some ISPs consider it "fair" if both ISPs take the shortest path out
> of their own netork to the other.  In this case both will strip MED
> before advertising to the IGP.  They will usually accept MED for the
> purpose of determining which of multiple routers to select at a
> peering point.

Right, and ALWAYS pick the peer that uses the IGP metric/MED allocation 
mechanism that results in the "lowest value", regardless of any real 
"metric".

> If ISP A has a much faster or less congested network than ISP B, ISP A
> may accept MED from ISP B and route based on MED.  If ISP B sets MED
> according to IGP cost, then a longer path might be taken through the
> ISP A network but the shortest path will be taken in ISP B's network
> in both directions.  This is often the case if B is a customer of A.

Likewise, if ISP A has congestion at a peering point or backbone link
they may muck with the MEDs such that "accepting peers" carry the traffic 
across their network and hand it off where ISP A considers the optimal 
local entry point, potentially changing network and bw/util 
characteristics of the peers networks without any notice or other 
indications, etc..

I think MEDs have their uses, but again, see my first comment above.
As always, YMMV.

-danny