Re: [Idr] draft-ietf-idr-rfc2796bis-01: what's the exact tie-breaking rules
Tony Przygienda <prz@xebeo.com> Sun, 07 November 2004 08:34 UTC
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Date: Sun, 07 Nov 2004 10:31:14 +0100
From: Tony Przygienda <prz@xebeo.com>
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To: Enke Chen <enke@bcn-inc.net>
Subject: Re: [Idr] draft-ietf-idr-rfc2796bis-01: what's the exact tie-breaking rules
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Enke Chen wrote: > Hi, Tony: > > In general these rules are necessary for route reflection to function > properly, > and they have been widely deployed for at least 6 or 7 years. > Hey Enke, that I know and my point was not that 'it does not work', it was rather request for clarficiation of the spec to reflect reality better, especially for ECMP ;-) > > > Please see my comments inline. > > > * To: idr at ietf.org > > * Subject: [Idr] draft-ietf-idr-rfc2796bis-01: what's the exact > tie-breaking rules > > * From: Tony Przygienda <prz at xebeo.com> > > * Date: Tue, 19 Oct 2004 09:55:24 +0200 > > > > Reading through bis of the RR draft stumbled over > > > > > > 9. Impact on Route Selection > > > > > > The BGP Decision Process Tie Breaking rules (Sect. 9.1.2.2, [1]) are > > modified as follows: > > > > > > If a route carries the ORIGINATOR_ID attribute, then in Step f) > > the ORIGINATOR_ID SHOULD be treated as the BGP Identifier of > > the BGP speaker that has advertised the route. > > > > This requirement probably should be a MUST. Route selection for IBGP > paths > must be consistent to avoid forwarding loops. Thus, routers within an > AS must > have a consistent view with respect to the BGP Identifier of the > router that > sources the route in the AS. (BGP Identifier impacts route selection.) > > Forwarding loop can occur if RR is used in a network and some routers > do not > follow this rule. > agreed. A sentence explaining what can or cannot happen if you have routers that do not understand ORIGINATOR_ID and therefore don't follow this rule would improve the spec IMHO. > > > > > > In addition, the following rule SHOULD be inserted between Steps > > f) and g): a BGP Speaker SHOULD prefer a route with the shorter > > CLUSTER_LIST length. The CLUSTER_LIST length is zero if a route > > does not carry the CLUSTER_LIST attribute. > > When the redundent RRs within a cluster are not configured with the > same cluster-id, > a RR may receive a route from its client directly, and from the other > RR. With the > right combinations of peering addresses, without this rule a RR may > select the one > from the other RR as the best path (and then withdraw its own > advertisement), and > routing may not converge. The rule is necessary to deal with this > corner case. > > As long as routing converges, forwarding loops wouldn't form even if some > routers do not follow this rule. (The routes being compared are of the > same > BGP Identifier, and thus the same next-hop anyway.) > yes, agreed of course. > > > > > > Now, the question comes up what to do with ECMP (which in many > implementations > > slightly deviates from 'official' spec last I checked) especially > with connection > > to the 'SHOULD' > > I am not aware of the existence of the "offical" spec for IBGP ECMP. > (Well I > hacked up the limited support for IBGP ECMP several years ago, and > that seemed > to have propagated ... :-( > > The first rule is important regardless of IBGP ECMP. The 2nd rule does > not seem > to have any bearing on IBGP ECMP. > > > > > > > Many implementations ignore the router_id for ECMP purposes & > load-balance > > that way and it is interesting now to think whether we can still do > that when > > > > a) one route has an ORIGINATOR_ID and the other none > > b) one route has an ORIGINATOR_ID_1 != ROUTER_ID_1 and > ORIGINATOR_ID_1 != ORIGINATOR_ID_2 > > BUT ROUTER_ID_1 == ORIGINATOR_ID_2 [I admit, pathological but > possible] > > > > and so on. > > > > > > Are we sure a mix of implementations old-style (ignoring the SHOULD) > and implementations > > of new-style (take ORIGINATOR_ID always [or only sometimes] as > ROUTER_ID) will not form > > loops with ECMP implementations ? > > > > The whole thing looks like another partial order in BGP route > ordering (the first infamous > > one lead to quite a couple of med-this and med-other ... options in > many vendors) and I > > wonder whether there is enough reason to do the same here. > > > > Same for CLUSTER_LIST length (albeit here I understand the scenario > that led to the > > rule & it makes sense). > > > ok, if the official stance is still 'there is no BGP ECMP spec' then we're in sync I guess since my questoin implicitly cannot be asked ;-) thanks much, Enke -- tony _______________________________________________ Idr mailing list Idr@ietf.org https://www1.ietf.org/mailman/listinfo/idr
- [Idr] draft-ietf-idr-rfc2796bis-01: what's the ex… Tony Przygienda
- Re: [Idr] draft-ietf-idr-rfc2796bis-01: what's th… Enke Chen
- Re: [Idr] draft-ietf-idr-rfc2796bis-01: what's th… Tony Przygienda