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

Alexander Vainshtein <vinesasha@yahoo.com> Tue, 28 November 2017 17:15 UTC

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Date: Tue, 28 Nov 2017 17:15:50 +0000
From: Alexander Vainshtein <vinesasha@yahoo.com>
Reply-To: "sasha@axerra.com" <sasha@axerra.com>
To: "bashandy@cisco.com" <bashandy@cisco.com>, 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>
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Subject: Re: Protecting SR policy midpoints (draft-bashandy-rtgwg-segment-routing-ti-lfa)
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Ahmed,I believe that the so-called protected Adj-SID simply means that if the link that it represents fails, it can be replaced with the Node-SID of the node at the remote end if the adjacency. It does not help at all if the downstream node fails.

Sent from Yahoo Mail on Android 
 
  On Tue, Nov 28, 2017 at 11:02, Ahmed Bashandy (bashandy)<bashandy@cisco.com> wrote:   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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