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Re: Protecting SR policy midpoints (draft-bashandy-rtgwg-segment-routing-ti-lfa)

"Ahmed Bashandy (bashandy)" <bashandy@cisco.com> Tue, 28 November 2017 17:01 UTC

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Date: Tue, 28 Nov 2017 09:01:51 -0800
From: "Ahmed Bashandy (bashandy)" <bashandy@cisco.com>
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To: Stewart Bryant <stewart.bryant@gmail.com>, Muthu Arul Mozhi Perumal <muthu.arul@gmail.com>, "sasha@axerra.com" <sasha@axerra.com>
CC: "rtgwg@ietf.org" <rtgwg@ietf.org>
Subject: Re: Protecting SR policy midpoints (draft-bashandy-rtgwg-segment-routing-ti-lfa)
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Stewart,

I am sure you are aware that ISIS and OSPF adj-SID advertisements 
indicate whether an adj-SID is protected or not. If the ingress router 
decided to use a protected adj-SID for a policy, then the protection of 
such adj-SID is within the policy.

Ahmed

On 11/28/2017 7:15 AM, Stewart Bryant wrote:
>
>
> On 28/11/2017 12:04, Ahmed Bashandy (bashandy) wrote:
>>
>> - The top label of incoming packet to node "S" is either a prefix SID 
>> owned by node "F" or an adjacency SID for (S,F)
>
> If it is an adjacency SID for (S,F) then you are violating the 
> original intent of the ingress PE which was to send the packet along 
> the path S->F. I really don't think you can blindly repair such a 
> packet since to do so violates the policy applied to the packet. You 
> have to do a policy check, and you have to make sure that the packet 
> is not subject to ECMP along the repair path since ECMP avoidance 
> might have been the intent of using the SR Adjacency in the first place.
>
> - Stewart

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