Re: [Lsr] A question about Mirror SID and its advertisement using IS-IS

"Les Ginsberg (ginsberg)" <> Tue, 19 June 2018 19:59 UTC

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From: "Les Ginsberg (ginsberg)" <>
To: Alexander Vainshtein <>, "" <>
CC: Michael Gorokhovsky <>, "" <>, "" <>
Thread-Topic: A question about Mirror SID and its advertisement using IS-IS
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Date: Tue, 19 Jun 2018 19:59:43 +0000
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Subject: Re: [Lsr] A question about Mirror SID and its advertisement using IS-IS
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Sasha -

Thanx for pointing this out.

Up until V13 of the draft the SID/Label Binding TLV had multiple use cases and there was introductory text in Section 2.4 which described these use cases.
All use cases other than SRMS were removed in V13 (principally the ERO types) as they were not in use and the introductory text was removed.

Support for Mirror SID was restored in V14 (thanx to Chris Bowers) but we never updated the introductory text in Section 2.4 to reflect this.

Attached are proposed diffs to address this gap. Hopefully this will resolve the issue for you.


From: Alexander Vainshtein <>
Sent: Tuesday, June 19, 2018 7:27 AM
Cc: Michael Gorokhovsky <>om>;;
Subject: A question about Mirror SID and its advertisement using IS-IS

Hi all,
I have a question about Mirror SID as defined in the SR Architecture<> draft and its advertisement defined in the IS-IS extensions for SR<> draft.

Mirror SID is defined in Section 5.1 of the SR Architecture draft as following:
5.1<>.1>.  IGP Mirroring Context Segment

   One use case for a Binding Segment is to provide support for an IGP
   node to advertise its ability to process traffic originally destined
   to another IGP node, called the Mirrored node and identified by an IP
   address or a Node-SID, provided that a "Mirroring Context" segment be
   inserted in the segment list prior to any service segment local to
   the mirrored node.

   When a given node B wants to provide egress node A protection, it

   advertises a segment identifying node's A context.  Such segment is

   called "Mirror Context Segment" and identified by the Mirror SID.

   The Mirror SID is advertised using the binding segment defined in SR

   IGP protocol extensions [I-D.ietf-isis-segment-routing-extensions<>] .

   In the event of a failure, a point of local repair (PLR) diverting

   traffic from A to B does a PUSH of the Mirror SID on the protected

   traffic.  B, when receiving the traffic with the Mirror SID as the

   active segment, uses that segment and processes underlying segments

   in the context of A.

Please note that these definitions do not mention SR Mapping Server or any such thing.

At the same time, the IS-IS Extensions for SR draft only mentions mirror context in Section 2.4 that says:

2.4<>.4>.  SID/Label Binding TLV

   The SID/Label Binding TLV MAY be originated by any router in an IS-IS


   The SID/Label Binding TLV is used to advertise prefixes to SID/Label

   mappings.  This functionality is called the Segment Routing Mapping

   Server (SRMS).  The behavior of the SRMS is defined in

2.4.1<>.1>.  Flags

   Flags: 1 octet field of following flags:

    0 1 2 3 4 5 6 7


   |F|M|S|D|A|     |



      F-Flag: Address Family flag.  If unset, then the Prefix carries an

      IPv4 Prefix.  If set then the Prefix carries an IPv6 Prefix.

      M-Flag: Mirror Context flag.  Set if the advertised SID

      corresponds to a mirrored context.  The use of the M flag is

      described in [I-D.ietf-spring-segment-routing<>]g>].


This seems to imply that mirrored context is related to the SRMS - but, to the best of my understanding, these are completely unrelated.
Please note also that the SR Architecture draft says that the Mirroring Context SID is a segment that identifies the node, and not any prefix. And, of course the SR Architecture draft does not describe usage of any flags.

What (if anything) did I miss?

Regards, and lots of thanks in advance,

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