Hi, Thanks for your response.
>> - Hence if the primary link fails, only "L1" will fail and L2 will not L1 _may_ fail, with high probability, but it may also not fail. If it does not fail, there is a second transitioning of the post-primary-failure link from FRR-backup (L2) to post-convergence link (L1), because L1 has a smaller metric. By "ambiguity", I meant that backup calculation taking SRLG into account is based on speculated topology, whereas computation of post-convergence path, ie, SPF, is based on actual topology. This seems needs reconciling since in TI-LFA the backup is by definition the post-convergence path, with a single path-transition after link-failure as the intended outcome. Do I understand correctly that the draft prefers to relax that expectation for SRLG? Thanks, Sikhi From: Ahmed Bashandy (bashandy) [mailto:basha...@cisco.com] Sent: 05 August 2017 01:19 To: Sikhivahan Gundu <sikhivahan.gu...@ericsson.com>; rtgwg@ietf.org Cc: rtgwg-cha...@ietf.org; pfr...@gmail.com; Stewart Bryant <stew...@g3ysx.org.uk> Subject: Re: I-D Action: draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt HI, All members of the same SRLG group are assumed to fail if one of them fails. Going back to you example - L1 is in the same SRLG group as the primary link while L2 is belongs a different group - Hence if the primary link fails, only "L1" will fail and L2 will not - Hence only L2 is candidate to become a backup path while L1 is not - Hence there is no ambiguity Thanks Ahmed On 8/1/2017 12:42 AM, Sikhivahan Gundu wrote: Hi, The draft mandates using "post-convergence path" as the backup path. It states one advantage, among others, of doing so as follows: "This .. helps to reduce the amount of path changes and hence service transients: one transition (pre-convergence to post-convergence) instead of two (pre-convergence to FRR and then post-convergence)". This suggests to me that the assumption here is that the post-convergence path can be uniquely determined in advance. However, SRLG introduces ambiguity. To illustrate the point, let us say a loop-free alternative has two options: one link (L1) is of the same metric value as the primary link and is also in the same SRLG as the primary; the second option (L2) is in a different SRLG and has higher metric. The actual post-convergence path would depend on whether or not L1 also failed along with the primary, so is not uniquely computed in advance. If TI-LFA picks L1, there might not be a guaranteed backup. If it picks L2, there'd be two link transitions because L2 would not be in a (strict) SPF- computed post-convergence path. A third option, of course, is to give up declaring that there is no TI-LFA backup, but it'd be preferable to have some backup than have none at all. What do the authors suggest for this situation? Thanks, Sikhi From: rtgwg [mailto:rtgwg-boun...@ietf.org] On Behalf Of Ahmed Bashandy (bashandy) Sent: 17 July 2017 12:56 To: rtgwg@ietf.org<mailto:rtgwg@ietf.org> Cc: rtgwg-cha...@ietf.org<mailto:rtgwg-cha...@ietf.org>; pfr...@gmail.com<mailto:pfr...@gmail.com>; Stewart Bryant <stew...@g3ysx.org.uk><mailto:stew...@g3ysx.org.uk> Subject: Fwd: I-D Action: draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt Hi, A new version of the ti-lfa draft has been posted to address Stewart Bryant's comments Thanks Ahmed -------- Original Message -------- Subject: I-D Action: draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt Date: Mon, 17 Jul 2017 00:19:37 -0700 From: internet-dra...@ietf.org<mailto:internet-dra...@ietf.org> Reply-To: internet-dra...@ietf.org<mailto:internet-dra...@ietf.org> To: <i-d-annou...@ietf.org><mailto:i-d-annou...@ietf.org> A New Internet-Draft is available from the on-line Internet-Drafts directories. Title : Topology Independent Fast Reroute using Segment Routing Authors : Ahmed Bashandy Clarence Filsfils Bruno Decraene Stephane Litkowski Pierre Francois Filename : draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt Pages : 12 Date : 2017-07-17 Abstract: This document presents Topology Independent Loop-free Alternate Fast Re-route (TI-LFA), aimed at providing protection of node and adjacency segments within the Segment Routing (SR) framework. This Fast Re-route (FRR) behavior builds on proven IP-FRR concepts being LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding (DLFA). It extends these concepts to provide guaranteed coverage in any IGP network. A key aspect of TI-LFA is the FRR path selection approach establishing protection over post-convergence paths from the point of local repair, dramatically reducing the operational need to control the tie-breaks among various FRR options. The IETF datatracker status page for this draft is: https://datatracker.ietf.org/doc/draft-bashandy-rtgwg-segment-routing-ti-lfa/ There are also htmlized versions available at: https://tools.ietf.org/html/draft-bashandy-rtgwg-segment-routing-ti-lfa-01 https://datatracker.ietf.org/doc/html/draft-bashandy-rtgwg-segment-routing-ti-lfa-01 A diff from the previous version is available at: https://www.ietf.org/rfcdiff?url2=draft-bashandy-rtgwg-segment-routing-ti-lfa-01 Please note that it may take a couple of minutes from the time of submission until the htmlized version and diff are available at tools.ietf.org. Internet-Drafts are also available by anonymous FTP at: ftp://ftp.ietf.org/internet-drafts/ _______________________________________________ I-D-Announce mailing list i-d-annou...@ietf.org<mailto:i-d-annou...@ietf.org> https://www.ietf.org/mailman/listinfo/i-d-announce Internet-Draft directories: http://www.ietf.org/shadow.html or ftp://ftp.ietf.org/ietf/1shadow-sites.txt
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