Re: [Lsr] SRLG usage in the IGP Flexible Algorithm draft
Peter Psenak <ppsenak@cisco.com> Fri, 01 May 2020 09:57 UTC
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To: Alexander Vainshtein <Alexander.Vainshtein@ecitele.com>, "shraddha@juniper.net" <shraddha@juniper.net>, "cfilsfil@cisco.com" <cfilsfil@cisco.com>, "ketant@cisco.com" <ketant@cisco.com>, "arkadiy.gulko@thomsonreuters.com" <arkadiy.gulko@thomsonreuters.com>
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From: Peter Psenak <ppsenak@cisco.com>
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Date: Fri, 01 May 2020 11:57:21 +0200
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Subject: Re: [Lsr] SRLG usage in the IGP Flexible Algorithm draft
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Alexander, On 30/04/2020 17:21, Alexander Vainshtein wrote: > Hi all, > > I have a question about the proposed usage of SRLG in the IGP Flexible > Algorithm <https://tools.ietf.org/html/draft-ietf-lsr-flex-algo-07> draft. > > This usage is defined Section 12 of the draft with the reference to the > SRLG exclude rule as following: > > > > 2. Check if any exclude SRLG rule is part of the Flex-Algorithm > > definition. If such exclude rule exists, check if the link is > > part of any SRLG that is also part of the SRLG exclude rule. If > > the link is part of such SRLG, the link MUST be pruned from the > > computation. > > This looks effectively undistinguishable from the usage of the exclude > Admin groups rule as described in the same Section 12 of the draft: > > 1. Check if any exclude rule is part of the Flex-Algorithm > > definition. If such exclude rule exists, check if any color that > > is part of the exclude rule is also set on the link. If such a > > color is set, the link MUST be pruned from the computation. > > From my POV, with such a definition, there is no need in the dedicated > “Exclude SRLG” rule as part of the specification of the Flexible > Algorithm, since such the SRLG Exclude rule can be replaced with a > matching Exclude All rule using Admin groups. there is one very important point. Maintaining the affinities is operationally complex. Some networks have already deployed SRLGs. If SRLG exclude with flex-algo gives people what they want, asking them to deploy affinities would be redundant. That's the main point. Simple use case: I have two SRLGs in the network. For some specific traffic I want to send the data via two independent streams. I want to avoid single SRLG failure to affect both streams. I define two flex-algos, each one excluding one SRLG. No need to define extra affinities. This is btw not an academical example, this has been requested by real users. > > I also think that such a usage of SRLG does not fit the needs of the > TI-LFA > <https://tools.ietf.org/html/draft-ietf-rtgwg-segment-routing-ti-lfa-03> > draft that considers an SRLG as a resource that fails when any of the > links/nodes comprising it fails. E.g., it says in Section 2: > > The Point of Local Repair (PLR), S, needs to find a node Q (a repair > > node) that is capable of safely forwarding the traffic to a > > destination D affected by the failure of the protected link L, a set > > of links including L (SRLG), or the node F itself. The PLR also > > needs to find a way to reach Q without being affected by the > > convergence state of the nodes over the paths it wants to use to > > reach Q: the PLR needs a loop-free path to reach Q. I see no conflict between the LFA draft and flex-algo one. If you do, please be precise where the conflict is. > > To me this suggests that SRLGs are only relevant when computing backup > paths for specific failures, e.g., an LFA for failure of a link hat > belongs to a specific SRLG must be computed in the topology from which > all the links belonging to the same SRLG are pruned. This understanding > matches RFC 4090 that states in Section 6.2 “Procedures for Backup Path > Computation”: SRLG is a mechanism to express fate share. I see no reason why SRLG could not be used for other than LFA purposes. thanks, Peter > > - The backup LSP cannot traverse the downstream node and/or link > > whose failure is being protected against. Note that if the PLR > > is the penultimate hop, node protection is not possible, and > > only the downstream link can be avoided. The backup path may be > > computed to be SRLG disjoint from the downstream node and/or > > link being avoided. > > If SRLGs are only relevant for computation of backup paths, it is not > clear to me if they should be part of the definition of a specific > Flexible Algorithm. > > What, if anything, did I miss? > > Regards, and lots of thanks in advance, > > Sasha > > Office: +972-39266302 > > Cell: +972-549266302 > > Email: Alexander.Vainshtein@ecitele.com > > > ___________________________________________________________________________ > > This e-mail message is intended for the recipient only and contains > information which is > CONFIDENTIAL and which may be proprietary to ECI Telecom. If you have > received this > transmission in error, please inform us by e-mail, phone or fax, and > then delete the original > and all copies thereof. > ___________________________________________________________________________
- [Lsr] SRLG usage in the IGP Flexible Algorithm dr… Alexander Vainshtein
- Re: [Lsr] SRLG usage in the IGP Flexible Algorith… Peter Psenak
- Re: [Lsr] SRLG usage in the IGP Flexible Algorith… Alexander Vainshtein
- Re: [Lsr] SRLG usage in the IGP Flexible Algorith… Peter Psenak
- Re: [Lsr] SRLG usage in the IGP Flexible Algorith… Alexander Vainshtein