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

Stewart Bryant <stewart.bryant@gmail.com> Tue, 28 November 2017 15:15 UTC

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Subject: Re: Protecting SR policy midpoints (draft-bashandy-rtgwg-segment-routing-ti-lfa)
To: "Ahmed Bashandy (bashandy)" <bashandy@cisco.com>, Muthu Arul Mozhi Perumal <muthu.arul@gmail.com>, "sasha@axerra.com" <sasha@axerra.com>
Cc: "rtgwg@ietf.org" <rtgwg@ietf.org>
References: <CAKz0y8wLYjkSO486w5WpSuDYV3Cjvgkv6887o9-Ky9o_ViWMrQ@mail.gmail.com> <210606893.1211556.1511362363266@mail.yahoo.com> <CAKz0y8xeYnqOjLxADVwndtOp8QQaPeQBiAO2TtnCi6pYfebONA@mail.gmail.com> <5A1D50E5.7030302@cisco.com>
From: Stewart Bryant <stewart.bryant@gmail.com>
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Date: Tue, 28 Nov 2017 15:15:07 +0000
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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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