Re: I-D Action: draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt

[Stewart Bryant <[e-mail]>]

On 07/08/2017 06:45, Sikhivahan Gundu wrote:
>
> 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?
>
>

Yes, that is a good point, in the event of an incomplete failure
of an SRLG, there may not be congruence between the
FRR path and the post convergence path. This certainly
needs further study.

   *
A--------//---------B
|                   |
|  *                | cost 2
C-------------------D
|                   |
|                   | cost 100
E-------------------F


AB + CD in same SRLG

TiLFA path is ACEFDB

Post convergence path is ACDB

In this case I think that the impact is just more SR hops in the
repair path than might be needed without the SRLG, but we do need to
be sure  that there are no pathological  cases in
topologies that lack the proposed congruence, and as
Sikhivahan notes this effect does need to be clarified in the
text.

- Stewart