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

"Ahmed Bashandy (bashandy)" <bashandy@cisco.com> Fri, 01 December 2017 18:02 UTC

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Date: Fri, 01 Dec 2017 10:02:55 -0800
From: "Ahmed Bashandy (bashandy)" <bashandy@cisco.com>
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To: "sasha@axerra.com" <sasha@axerra.com>, Stewart Bryant <stewart.bryant@gmail.com>, Muthu Arul Mozhi Perumal <muthu.arul@gmail.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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Not necessarily. However even if the intention of marking the adj-SID as 
protected is to protect against the failure of the link but not the far 
end of the link, then it is the responsibility of operator who 
instantiated the policy creator to know what is the type of protection 
that he/she configured on the link.


Ahmed

On 11/28/2017 9:15 AM, Alexander Vainshtein wrote:
> 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 
> <https://overview.mail.yahoo.com/mobile/?.src=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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