RE: Is TI-LFA compatible with the default SR algorithm?

<> Thu, 14 June 2018 10:56 UTC

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To: Alexander Vainshtein <>, "" <>
CC: Stewart Bryant <>, Michael Gorokhovsky <>, "" <>, "" <>, "" <>
Subject: RE: Is TI-LFA compatible with the default SR algorithm?
Thread-Topic: Is TI-LFA compatible with the default SR algorithm?
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Date: Thu, 14 Jun 2018 10:56:30 +0000
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Hi Sasha,

Could you elaborate on :" I strongly suspect that it is not so, and that these mechanisms are only compatible with the Strict-SPF. (Actually, I can provide an example that confirms this suspicion.)" ?


From: spring [] On Behalf Of Alexander Vainshtein
Sent: Wednesday, June 13, 2018 17:00
Cc: Stewart Bryant; Michael Gorokhovsky;;;
Subject: [spring] Is TI-LFA compatible with the default SR algorithm?

Hi all,
I have looked up Section 3.1.1 "Prefix-SID Algorithm" of the Segment Routing Architecture<> draft (already In the RFC Editor queue) and found there the following statement (the relevant part is highlighted):

This document defines two algorithms:

   o  "Shortest Path": this algorithm is the default behavior.  The
      packet is forwarded along the well-known ECMP-aware SPF algorithm
      employed by the IGPs.  However it is explicitly allowed for a
      midpoint to implement another forwarding based on local policy.
      The "Shortest Path" algorithm is in fact the default and current
      behavior of most of the networks where local policies may override
      the SPF decision.

   o  "Strict Shortest Path (Strict-SPF)": This algorithm mandates that
      the packet is forwarded according to ECMP-aware SPF algorithm and
      instructs any router in the path to ignore any possible local
      policy overriding the SPF decision.  The SID advertised with
      Strict-SPF algorithm ensures that the path the packet is going to
      take is the expected, and not altered, SPF path.  Note that Fast
      Reroute (FRR) [RFC5714<>] mechanisms are still compliant with the
      Strict Shortest Path.  In other words, a packet received with a
      Strict-SPF SID may be rerouted through a FRR mechanism.

At the same time, the TI-LFA draft<> discusses protection of active Prefix-SIDs (e.g., in Section 3 that discusses P-Space and Q-space) but, to the best of my understanding, does not mention algorithms that form the context of these SIDs.

My question to the authors of the TI-LFA draft is:

Are the mechanisms defined in the draft (and examples discussed in Section 4) applicable to Prefix-SIDs associated with the default forwarding algorithm as defined in the Segment Routing Architecture draft?

I strongly suspect that it is not so, and that these mechanisms are only compatible with the Strict-SPF. (Actually, I can provide an example that confirms this suspicion.)

Do I miss something substantial here?

Regards, and lots of thanks in advance,

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