Hiya, I was wondering if someone had done any or some research on this before... 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. 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. I do not trust all the BGP implementations around, and we consider changing the default outbound, with MEDs of course still available on request. Alexander
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
On Jan 3, 2006, at 1:03 AM, Daniel Roesen wrote:
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 recall some earlier implementations from "well known" vendors that had varying behavior for MED processing as well. Fortunately, the update to RFC 1771: http://www.ietf.org/internet-drafts/draft-ietf-idr-bgp4-26.txt is considerably more explicit about this behavior, as well as a slew of other previously-left-to-the-implementation items ironed out through a great deal of implementation and deployment experience. The "BGP Experience" and "BGP MED Considerations" Internet- drafts provide a good bit of additional insight into some of these behaviors. -danny
And just to update, those drafts have made it into RFC 4271 http://www.ietf.org/rfc/rfc4271.txt -----Original Message----- From: owner-nanog@merit.edu [mailto:owner-nanog@merit.edu] On Behalf Of Danny McPherson Sent: Tuesday, January 03, 2006 3:49 PM To: nanog@nanog.org Subject: Re: metric 0 vs 'no metric at all' On Jan 3, 2006, at 1:03 AM, Daniel Roesen wrote:
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 recall some earlier implementations from "well known" vendors that had varying behavior for MED processing as well. Fortunately, the update to RFC 1771: http://www.ietf.org/internet-drafts/draft-ietf-idr-bgp4-26.txt is considerably more explicit about this behavior, as well as a slew of other previously-left-to-the-implementation items ironed out through a great deal of implementation and deployment experience. The "BGP Experience" and "BGP MED Considerations" Internet- drafts provide a good bit of additional insight into some of these behaviors. -danny
:-> "Alexander" == Alexander Koch <efraim@clues.de> writes: > Hiya, > I was wondering if someone had done any or some research on > this before... 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. 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. > I do not trust all the BGP implementations around, and we > consider changing the default outbound, with MEDs of course > still available on request. Cisco has a knob for this: http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123tcr/123ti... default is treating missing med as 0, which tends to surprise peers/customers who want to send med only on a few routes (i.e., if you want to set a med on a route, always set it on all your bgp sessions, because you can't tell if your neighbor has set "missing-as-worst" or not). Pf > Alexander -- ------------------------------------------------------------------------------- Pierfrancesco Caci | Network & System Administrator - INOC-DBA: 6762*PFC p.caci@seabone.net | Telecom Italia Sparkle - http://etabeta.noc.seabone.net/ Linux paperino 2.4.20 #1 Mon Dec 2 17:02:14 CET 2002 i686 GNU/Linux
On Tue, 3 Jan 2006, Alexander Koch wrote:
I was wondering if someone had done any or some research on this before... 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. 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.
I do not trust all the BGP implementations around, and we consider changing the default outbound, with MEDs of course still available on request.
i had this some time ago, cant remember where but i found some answers... basically no metric is undefined, and can be handled either as all-zero or all-ones or anything you want really the best practice was to set med manually to ensure the expected behaviour occurred. Steve
participants (6)
-
Alexander Koch -
Daniel Roesen -
Danny McPherson -
Doug Marschke -
Pierfrancesco Caci -
Stephen J. Wilcox