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Re: metric 0 vs 'no metric at all'
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: email@example.com -- dr@IRCnet -- PGP: 0xA85C8AA0