Muthu – I am glad we are in sync.
As to your suggested additional text. RFC 7775 (like its predecessor RFC 5302) is not specifying what “SPF” algorithm is being applied nor how it is calculated. It is only specifying the route type preference that MUST be used independent of the type of SPF. In that regard, the statement “Note that all types of routes listed for a given preference are treated equally.” is stating exactly what is in scope for the RFC. So I do not think your suggested additional text should be added to RFC 7775. Les From: Muthu Arul Mozhi Perumal <[email protected]> Sent: Monday, February 21, 2022 10:16 PM To: Les Ginsberg (ginsberg) <[email protected]> Cc: lsr <[email protected]> Subject: Re: [Lsr] Preference among IS-IS IPv6 route types Hi Les, Thanks for your explanation -- quite helpful. Please see inline.. On Fri, Feb 18, 2022 at 7:38 PM Les Ginsberg (ginsberg) <[email protected]<mailto:[email protected]>> wrote: Muthu – Let me try to be more complete in my response. What RFC 7775 is addressing is defining the route preference between different route types. It is necessary for interoperability that all implementations use the same preference rules in this regard. (One of the motivations for the RFC was a real world interoperability issue that occurred because of inconsistency in this regard – see Appendix A.) Understood.. Within the preference groups defined in Sections 3.3 and 3.4, an implementation has to apply the criteria used in the SPF algorithm. If this is the well known Dijkstra lowest cost algorithm described in Annex C.2 of ISO 10589, then within the set of routes with highest preference we choose the path(s) with lowest cost. Makes sense and that's why I pasted a part of the foll. text from RFC5308 in my original mail: <snip> If multiple paths have the same best preference, then selection occurs based on metric. Any remaining multiple paths SHOULD be considered for equal-cost multi-path routing if the router supports this; otherwise, the router can select any one of the multiple paths. </snip> Since RFC7775 only updates RFC5308 (and not replaces it), I believe the rule also applies to RFC7775. The text in RFC7775 that is slightly confusing (at least for me) is: <snip> Note that all types of routes listed for a given preference are treated equally. </snip> Would sound better to replace it with what you said: "Within the preference groups defined in Sections 3.3 and 3.4, an implementation has to apply the criteria used in the SPF algorithm." If there are multiple paths with the same cost, an implementation may install any or all of them in forwarding even if all routes are NOT of the same type. In your example, so long as the cost to reach the destination using the L1 intra-area route is the same as the cost to reach the destination via the L1 External route, use of either path will NOT result in looping. And one node could install both paths – another could install only one path – and still no looping would occur. Lack of use of ECMP might result in link utilization that is not to the liking of a customer, but no interoperability issue will occur. So the prioritization you mention below is NOT required to avoid looping. Fully agree.. Regards, Muthu Les From: Muthu Arul Mozhi Perumal <[email protected]<mailto:[email protected]>> Sent: Thursday, February 17, 2022 11:59 PM To: Les Ginsberg (ginsberg) <[email protected]<mailto:[email protected]>> Cc: lsr <[email protected]<mailto:[email protected]>> Subject: Re: [Lsr] Preference among IS-IS IPv6 route types Hi Les, If we do ECMP, we'll have a traffic loop in the topology described in Appendix A of RFC7775 b/w R1 and R2, assuming all routes are L1, right? Seems prioritizing one of the routes (intra-area vs external) or honouring the metric is required for avoiding this loop.. Regards, Muthu On Thu, Feb 17, 2022 at 9:46 PM Les Ginsberg (ginsberg) <[email protected]<mailto:[email protected]>> wrote: Muthu – Use of Equal Cost Multipath (ECMP) is commonplace. Les From: Muthu Arul Mozhi Perumal <[email protected]<mailto:[email protected]>> Sent: Thursday, February 17, 2022 7:51 AM To: Les Ginsberg (ginsberg) <[email protected]<mailto:[email protected]>> Cc: lsr <[email protected]<mailto:[email protected]>> Subject: Re: [Lsr] Preference among IS-IS IPv6 route types Hi Les, Thanks for your response. Please see inline.. On Thu, Feb 17, 2022 at 8:56 PM Les Ginsberg (ginsberg) <[email protected]<mailto:[email protected]>> wrote: Muthu – RFC 7775 is defining preference rules between routes of different types – it is NOT discussing preference rules within a (set of) route types that have the same preference. Ok, but RFC7775 says "Note that all types of routes listed for a given preference are treated equally". How is that to be interpreted when there is an L1 intra-area route of metric a and an L1 external route of metric b for the same IPv6 prefix during comparison? Regards, Muthu Such a discussion is out of scope. Use of “lowest cost” is part of the well known Dijkstra Shortest Path First (SPF) algorithm – though there are many example of constrained SPF calculations that incorporate attributes other than cost in the choice of “best path”. All of this is out of scope for RFC 7775. Les From: Lsr <[email protected]<mailto:[email protected]>> On Behalf Of Muthu Arul Mozhi Perumal Sent: Thursday, February 17, 2022 6:49 AM To: lsr <[email protected]<mailto:[email protected]>> Subject: [Lsr] Preference among IS-IS IPv6 route types Hi, Need some clarification on the preference among IS-IS IPv6 route types described in RFC7775 section 3.4 and RFC5308 section 5. RFC7775 places L1 intra-area routes and L1 external routes at the same preference level and says that all types of routes listed for a given preference are treated equally. There is no mention of metric. <snip> This document defines the following route preferences for IPv6 routes advertised in TLVs 236 or 237. Note that all types of routes listed for a given preference are treated equally. 1. L1 intra-area routes; L1 external routes 2. L2 intra-area routes; L2 external routes; L1->L2 inter-area routes; L1-L2 external routes; L2-L2 inter-area routes 3. L2->L1 inter-area routes; L2->L1 external routes; L1->L1 inter- area routes </snip> RFC5308 however says: <snip> If multiple paths have the same best preference, then selection occurs based on metric. </snip> It is not clear whether metric is to be used for selection among L1 intra-area and external routes or is to be used for selection only with a given route type. Can someone please clarify? Regards, Muthu
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