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Re: metric 0 vs 'no metric at all'
From: Daniel Roesen <dr () cluenet de>
Date: Tue, 3 Jan 2006 09:03:08 +0100


On Tue, Jan 03, 2006 at 08:21:43AM +0100, Alexander Koch wrote:
I was wondering if someone had done any or some research on
this before...

Yup, when troubleshooting the ERXes former wrong handling of
"no MED". :-)

basically I am not sure with all the many
implementations of BGP and all the vendors if and what those
will do when they see a metric of 0 and no metric.

"no MED" is supposed to be equal to "MED=0". Some BGP implementations
(e.g. ERX / Juniper E-Series) have knobs like "bgp bestpath
missing-as-worst" to fiddle with that.

RFC1771 states:

         d) MULTI_EXIT_DISC (Type Code 4):

            This is an optional non-transitive attribute that is a four
            octet non-negative integer. The value of this attribute may
            be used by a BGP speaker's decision process to discriminate
            among multiple exit points to a neighboring autonomous
            system.

   The MULTI_EXIT_DISC attribute may be used on external (inter-AS)
   links to discriminate among multiple exit or entry points to the same
   neighboring AS.  The value of the MULTI_EXIT_DISC attribute is a four
   octet unsigned number which is called a metric.  All other factors
   being equal, the exit or entry point with lower metric should be
   preferred.  If received over external links, the MULTI_EXIT_DISC
   attribute may be propagated over internal links to other BGP speakers
   within the same AS.  The MULTI_EXIT_DISC attribute is never
   propagated to other BGP speakers in neighboring AS's.

So the spec is fuzzy about how "no MED vs. MED=0" should be treated, but
vendors seem to largely agree to "no MED == MED 0". I know of no
deviation, except the old ERX bug which got fixed (ERX treated "no MED"
as best, even better than MED=0 - contrary to documentation).

I am not an expert knowing the actual protocol's messages exchanged,
but I see some routes with nothing in the metric field on the
various show commands, and some have explicit '0' metric.

MED is an optional attribute which may be present or not.


Best regards,
Daniel

-- 
CLUE-RIPE -- Jabber: dr () cluenet de -- dr () IRCnet -- PGP: 0xA85C8AA0


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