Hi Ahmed, Thanks for clearing that up.
Under section, 11. Advantages of using the expected post-convergence path during FRR, at the beginning of page 17, it says As an example, if the operator has implemented a protection against a node failure, the expected post-convergence path used during FRR will be the one considering that the node has failed. However, even if a single link is failing or a set of links is failing (instead of the full node), the node-protecting post- convergence path will be used. Now, Cisco IOS-XR follows this rule, with no issues. But the Nokia SROS does things a bit differently. It calculates the post-convergence path first, then trims down the SRLG and nodes. The problem is, sometimes it ends up with no protection path at all. They're claiming that using Cisco's approach to TI-LFA can cause issues with seamless BFD during an incident. Unfortunately, I don't have the details on what kind of trouble they're talking about. Would it be accurate to assert that their implementation is non-compliant? Regards, Hemant On Tue, Jan 30, 2024 at 3:43 AM Ahmed Bashandy <[email protected]> wrote: > > Thanks for the comment > > Regarding the paragraph from section 6 that you are referring to, this > paragraph was part of an example and not a recommendation. > > RFC5286 and RFC7490 are referred to in > draft-ietf-rtgwg-segment-routing-ti-lfa. So IMO it would be redundant to > add any recommendation in draft-ietf-rtgwg-segment-routing-ti-lfa that > already exist in the RFCs. > > > Thanks > > Ahmed > > > On 12/30/23 2:46 PM, Hemant Sharma wrote: > > Hello, > > The RFC 7490, 5.2.2. Selecting Repair Paths, provides a recommendation > on how to choose a PQ node when multiple options are available. > > such as, > "For consistency in behavior, it is RECOMMENDED that the member of the set > of routers {PQ} with the lowest cost S->P be the default choice for P." > > or > > "As described in [RFC5286], always selecting a PQ node that is downstream > to the destination with respect to the repairing node prevents the > formation of loops when the failure is worse than expected. The use of > downstream nodes reduces the repair coverage, and operators are advised to > determine whether adequate coverage is achieved before enabling this > selection feature." > > whereas, in the current draft, 6. TI-LFA Repair path, there is only a > subtle reference to the selection of a specific P node, as shown below > > * P(S, N1) intersection with PCPath is [N2, R1], > *R1 being the deeper downstream node in PCP, it can be assumed to be > used as P node* > (this is an example and an implementation could use a different > strategy to choose the P node). > > However, in Section 6.2 on FRR path using a PQ node, there is no explicit > mention of the scenario when there are multiple PQ nodes along the > post-convergence path. > > > I believe it is worthwhile to include similar recommendations here for the > sake of completeness, either in the same section or creating another one, > copying this part from above. > > "For consistency in behavior, it is RECOMMENDED that the member of the set > of routers {PQ} with the lowest cost S->P be the default choice for P." > or > "As described in [RFC5286], always selecting a PQ node that is downstream > to the destination with respect to the repairing node prevents the > formation of loops when the failure is worse than expected. The use of > downstream nodes reduces the repair coverage, and operators are advised to > determine whether adequate coverage is achieved before enabling this > selection feature." > > > Please confirm if I have correctly understood this. > > Thanks! > > Regards, > Hemant Sharma > > > _______________________________________________ > rtgwg mailing [email protected]https://www.ietf.org/mailman/listinfo/rtgwg > >
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