Muthu – Use of Equal Cost Multipath (ECMP) is commonplace.
Les From: Muthu Arul Mozhi Perumal <[email protected]> Sent: Thursday, February 17, 2022 7:51 AM 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 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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