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

Your answer did not address the issue below, which is one of a class of 
issues related to SRLG.

- Stewart

On 07/08/2017 19:23, Ahmed Bashandy (bashandy) wrote:
> See my reply to Sikhi
>
> Thanks
>
> Ahmed
>
>
> On 8/7/2017 2:13 AM, Stewart Bryant wrote:
>>
>>
>>
>> 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
>>
>>
>>
>>
>
>
>
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