North American Network Operators Group

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Re: Communities BCP [was: RE: BGP Path Filtering]

  • From: Danny McPherson
  • Date: Fri May 16 19:14:00 2003

On 5/16/03 4:50 PM, "Leo Bicknell" <[email protected]> wrote:

> Since this started off talking about customers, I must also point
> out:
> Many customers want MEDs to be honored.  They don't want their
> transit provider, who they are paying money to carry the bits where
> they want them to be, to be closest exiting to them.

They want another feature, and someone sold them with the "we'll accept MEDs
from you and not do closest-exit like your peers" line.  They forgot to
mention route oscillation might occur, or if the customer does any form of
intelligent aggregation MEDs are probably broken, or if things aren't
tweaked perfectly things can be worse than closest-exit, etc.., etc..

I've seen MEDs work "as advertised" (no pun intended ;-) in several
configurations, but in general I've seen them break things first-hand in 3
very large networks.

> I would think the most common configuration in large networks is to
> honor meds from customers, not honor them from peers, but send them
> to peers.  Note I said common, and not majority.

"most common" would imply majority.  I doubt that's the case.

> At the end of the day the best advice I have is to be mindful of
> what you do with MEDS.  Too many people like to ignore them, and
> that is a problem.  Don't want to send them, fine, but be religous
> about getting rid of them everywhere or you'll cause problems.

You'll fix more problems then you'll cause, guaranteed.  Folks are
mesmerized by tuning yet another knob and assuming everything is dandy
because they can ping some destination.  It's more than that, much more.
Take the update packing comments earlier as yet another effect.

> Want to use them, fine, but make sure they mean something and
> aren't garbage. 

How exactly do you make sure you're peers MEDs aren't garbage?  Accept
more-specifics?  Configure and maintain their routers?  And if they're
multi-homed to another network and accept MEDs from them do you simply
configure "always-compare-med" in your router and assume things are great?
They're probably not.

Most folks seem to have enough of a problem deriving their own.

> Ignoring the consequences of meds in  your config
> choices _will_ cause you problems.

Yes, that's why you remove them and understand the consequences of not doing
so.  RFC 3345 provides one such example, though I'm certain that aggregation
breaks MEDs more often than anything.

MEDs will work to influence route selection, no doubt, but you'd better read
the fine print and between the lines.