I think the P space calculation in Section 6 is the extended P-space as defined in RFC7490, i.e. path to R1 is no longer ECMP once N2 is selected as next hop. See also ecmp path to node C In figure 1 of that RFC7490. Dirk ________________________________ Van: rtgwg <[email protected]> namens Yasuhiro Ohara <[email protected]> Verzonden: zondag 24 maart 2024 6:54 Aan: [email protected] <[email protected]> CC: Yasuhiro Ohara <[email protected]> Onderwerp: Question for TI-LFA
[Some people who received this message don't often get email from [email protected]. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ] CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information. Hi, I have a question for the draft-ietf-rtgwg-segment-routing-ti-lfa-13. I wonder if it needs a fix. In the I-D, the Section 3. "Terminology" defines the P-space as the following. > The P-space P(R,X) of a router R with regard to a resource X (e.g. a > link S-F, a node F, or a SRLG) is the set of routers reachable from R > using the pre-convergence shortest paths without any of those paths > (including equal-cost path splits) transiting through X. The Figure 1 (Section 6) in the same I-D, the resulting P(S, N1) includes R1, but one of the S's ECMPs to R1 includes N1. S's ECMPs to R1: [(S-N1-R1), (S-N2-R1)]. How can we include R1 in the P(S,N1), given the P-space definition? My current guess is that P-space definition needs additional explanation on the ECMP part. My guess for the correct definition is: A router (say 'U') can be included in the P(R,X) as long as the R can exclude all the nexthops possibly transiting through X. I think we are implicitly assuming that S can eliminate sending through N1 to R1 by itself, and so the R1 can be include in P(S,N1) in Section 6. As a search for other problematic example, we can manipulate(generate artificially) the topology such that S's ECMPs to R1 consist of: S-X-A-R1 S-B-R1 S-C-X-R1 S-D-E-R1 S-D-X-R1 In this case, R1 can be included only if S can eliminate the X, C, D from the nexthops to R1. S-X-A-R1 (NG, easily avoidable) S-B-R1 (OK) S-C-X-R1 (NG, avoidable after path calculation) S-D-E-R1 (NG, hard to avoid unless we compute ECMP from D to R1) S-D-X-R1 (NG, hard to avoid unless we compute ECMP from D to R1) The current definition seems to worry about inclusion of D nexthop case, and contradicts with the raised example which includes B nexthop case. By the way, I think Q-space definition is correct as is in the current version. Best regards, Yasu _______________________________________________ rtgwg mailing list [email protected] https://www.ietf.org/mailman/listinfo/rtgwg
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