Hi all,
I have looked up Section 3.1.1 "Prefix-SID Algorithm" of the Segment Routing
Architecture<https://tools.ietf.org/html/draft-ietf-spring-segment-routing-15>
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<https://tools.ietf.org/html/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<https://tools.ietf.org/html/draft-bashandy-rtgwg-segment-routing-ti-lfa-04>
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,
Sasha
Office: +972-39266302
Cell: +972-549266302
Email: alexander.vainsht...@ecitele.com
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