[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-muley-pwe3-redundancy) 00 01 02 03 04 05 06 07 08 09 RFC 6718

Network Working Group                                           P. Muley
Internet-Draft                                               M. Aissaoui
Intended status: Informational                                  M. Bocci
Expires: November 4, 2012                                 Alcatel-Lucent
                                                             May 3, 2012


                         Pseudowire Redundancy
                     draft-ietf-pwe3-redundancy-08

Abstract

   This document describes a framework comprised of a number of
   scenarios and associated requirements for pseudowire (PW) redundancy.
   A set of redundant PWs is configured between provider edge (PE) nodes
   in single -segment PW applications, or between terminating PE (T-PE)
   nodes in multi-segment PW applications.  In order for the PE/T-PE
   nodes to indicate the preferred PW to use for forwarding PW packets
   to one another, a new PW status is required to indicate the
   preferential forwarding status of active or standby for each PW in
   the redundancy set.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 4, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the



Muley, et al.           Expires November 4, 2012                [Page 1]


Internet-Draft                PW Redundancy                     May 2012


   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Reference Models . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  PE Architecture  . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  PW Redundancy Network Reference Scenarios  . . . . . . . .  5
       3.2.1.  Single Multi-Homed CE  . . . . . . . . . . . . . . . .  5
       3.2.2.  Multiple Multi-Homed CEs . . . . . . . . . . . . . . .  7
       3.2.3.  Single-Homed CE With MS-PW Redundancy  . . . . . . . .  8
       3.2.4.  PW Redundancy Between MTU-s in H-VPLS  . . . . . . . .  9
       3.2.5.  PW Redundancy Between VPLS Network Facing PEs
               (n-PEs)  . . . . . . . . . . . . . . . . . . . . . . . 11
       3.2.6.  Redundancy in a VPLS Bridge Module Model . . . . . . . 12
   4.  Generic PW Redundancy Requirements . . . . . . . . . . . . . . 13
     4.1.  Protection Switching Requirements  . . . . . . . . . . . . 13
     4.2.  Operational Requirements . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   7.  Major Contributing Authors . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16













Muley, et al.           Expires November 4, 2012                [Page 2]


Internet-Draft                PW Redundancy                     May 2012


1.  Introduction

   The objective of PW redundancy is to enable redundant attachment
   circuits (ACs), provider edge nodes (PEs), and pseudowires (PWs) to
   eliminate single points of failure in the path of an emulated
   service.  This is achieved while ensuring that only one active path
   exists between a pair of customer edge nodes (CEs).

   In single-segment PW (SS-PW) applications, protection for the PW is
   provided by the packet switched network (PSN) layer.  This may be a
   resource reservation protocol with traffic engineering (RSVP-TE)
   labeled switched path (LSP) with a fast-reroute (FRR) backup or an
   end-to-end backup LSP.  It is assumed that these mechanisms can
   restore PSN connectivity rapidly enough to avoid triggering
   protection by PW redundancy.  PSN protection mechanisms cannot
   protect against the failure of a PE node or the failure of the remote
   AC.  Typically, this is supported by dual-homing a customer edge (CE)
   node to different PE nodes which provide a pseudowire emulated
   service across the PSN.  A set of PW mechanisms is therefore required
   that enables a primary and one or more backup PWs to terminate on
   different PE nodes.

   In multi-segment PW (MS-PW) applications, PSN protection mechanisms
   cannot protect against the failure of a switching PE (S-PE).  A set
   of mechanisms that support the operation of a primary and one or more
   backup PWs via a different set of S-PEs is therefore required.  The
   paths of these PWs are diverse in the sense that they are switched at
   different S-PE nodes.

   In both of these applications, PW redundancy is important to maximise
   the resiliency of the emulated service.

   This document describes the framework for these applications and its
   associated operational requirements.  The framework utilizes a new PW
   status, called the Preferential Forwarding Status of the PW.  This is
   separate from the operational states defined in RFC4447 [RFC4447].
   The mechanisms for PW redundancy are modeled on general protection
   switching principles.


2.  Terminology

   o  Up PW: A PW which has been configured (label mapping exchanged
      between PEs) and is not in any of the PW defect states specified
      in [RFC4447].  Such a PW is is available for forwarding traffic.

   o  Down PW: A PW that has either not been fully configured, or has
      been configured and is in any one of the PW defect states



Muley, et al.           Expires November 4, 2012                [Page 3]


Internet-Draft                PW Redundancy                     May 2012


      specified in [RFC4447].  Such a PW is not available for forwarding
      traffic.

   o  Active PW: An UP PW used for forwarding user, OAM and control
      plane traffic.

   o  Standby PW: An UP PW that is not used for forwarding user traffic
      but may forward OAM and specific control plane traffic.

   o  PW Endpoint: A PE where a PW terminates on a point where native
      service processing is performed, e.g., A single-segment PW (SS-PW)
      PE, a multi-segment pseudowire (MS-PW) terminating PE (T-PE), or a
      hierarchical VPLS MTU-s or PE-rs.

   o  Primary PW: The PW which a PW endpoint activates (i.e. uses for
      forwarding) in preference to any other PW when more than one PW
      qualifies for the active state.  When the primary PW comes back up
      after a failure and qualifies for the active state, the PW
      endpoint always reverts to it.  The designation of primary is
      performed by local configuration for the PW at the PE and is only
      required when revertive behaviour is used.

   o  Secondary PW: When it qualifies for the active state, a secondary
      PW is only selected if no primary PW is configured or if the
      configured primary PW does not qualify for active state (e.g., is
      DOWN).  By default, a PW in a redundancy PW set is considered
      secondary.  There is no revertive mechanism among secondary PWs.

   o  Revertive protection switching: Traffic will be carried by the
      primary PW if it is UP and a wait-to-restore timer expires and the
      primary PW is made the active PW.

   o  Non-revertive protection switching: Traffic will be carried by the
      last PW selected as a result of previous active PW entering the
      operationally DOWN state.

   o  Manual selection of a PW: The ability to manually select the
      primary/secondary PWs.

   o  MTU-s: A hierarchical virtual private LAN service multi-tenant
      unit switch, as defined in RFC4762 [RFC4762].

   o  PE-rs: A hierarchical virtual private LAN service switch, as
      defined in RFC4762.

   o  n-PE: A network facing provider edge node, as defined in RFC4026
      [RFC4026].




Muley, et al.           Expires November 4, 2012                [Page 4]


Internet-Draft                PW Redundancy                     May 2012


   This document uses the term 'PE' to be synonymous with both PEs as
   per RFC3985[RFC3985] and T-PEs as per RFC5659 [RFC5659].

   This document uses the term 'PW' to be synonymous with both PWs as
   per RFC3985 and SS-PWs, MS-PWs, and PW segments as per RFC5659.


3.  Reference Models

   The following sections show the reference architecture of the PE for
   PW redundancy and the usage of the architecture in different
   topologies and applications.

3.1.  PE Architecture

   Figure 1 shows the PE architecture for PW redundancy when more than
   one PW in a redundant set is associated with a single AC.  This is
   based on the architecture in Figure 4b of RFC3985 [RFC3985].  The
   forwarder selects which of the redundant PWs to use based on the
   criteria described in this document.

              +----------------------------------------+
              |                PE Device               |
              +----------------------------------------+
     Single   |                 |        Single        | PW Instance
      AC      |                 +      PW Instance     X<===========>
              |                 |                      |
              |                 |----------------------|
      <------>o                 |        Single        | PW Instance
              |    Forwarder    +      PW Instance     X<===========>
              |                 |                      |
              |                 |----------------------|
              |                 |        Single        | PW Instance
              |                 +      PW Instance     X<===========>
              |                 |                      |
              +----------------------------------------+

                Figure 1: PE Architecture for PW redundancy

3.2.  PW Redundancy Network Reference Scenarios

   This section presents a set of reference scenarios for PW redundancy.

3.2.1.  Single Multi-Homed CE

   The following figure illustrates an application of single segment
   pseudowire redundancy.  This scenario is designed to protect the
   emulated service against a failure of one of the PEs or ACs attached



Muley, et al.           Expires November 4, 2012                [Page 5]


Internet-Draft                PW Redundancy                     May 2012


   to the multi-homed CE.  Protection against failures of the PSN
   tunnels is provided using PSN mechanisms such as MPLS fast reroute,
   so that these failures do not impact the PW.

   CE1 is dual-homed to PE1 and PE3.  A dual homing control protocol,
   the details of which are outside the scope of this document, selects
   which AC CE1 should use to forward towards the PSN, and which PE (PE1
   or PE3) should forward towards CE1.

            |<-------------- Emulated Service ---------------->|
            |                                                  |
            |          |<------- Pseudo Wire ------>|          |
            |          |                            |          |
            |          |    |<-- PSN Tunnels-->|    |          |
            |          V    V                  V    V          |
            V    AC    +----+                  +----+     AC   V
      +-----+    |     | PE1|==================|    |     |    +-----+
      |     |----------|....|...PW1.(active)...|....|----------|     |
      |     |          |    |==================|    |          | CE2 |
      | CE1 |          +----+                  |PE2 |          |     |
      |     |          +----+                  |    |          +-----+
      |     |          |    |==================|    |
      |     |----------|....|...PW2.(standby)..|    |
      +-----+    |     | PE3|==================|    |
                 AC    +----+                  +----+

              Figure 2: PW Redundancy with One Multi-Homed CE

   In this scenario, only one of the PWs should be used for forwarding
   between PE1 / PE3, and PE2.  PW redundancy determines which PW to
   make active based on the forwarding state of the ACs so that only one
   path is available from CE1 to CE2.

   Consider the example where the AC from CE1 to PE1 is initially active
   and the AC from CE1 to PE3 is initially standby.  PW1 is made active
   and PW2 is made standby in order to complete the path to CE2.

   On failure of the AC between CE1 and PE1, the forwarding state of the
   AC on PE3 transitions to Active.  The preferential forwarding state
   of PW2 therefore needs to become active, and PW1 standby, in order to
   reestablish connectivity between CE1 and CE2.  PE3 therefore uses PW2
   to forward towards CE2, and PE2 uses PW2 instead of PW1 to forward
   towards CE1.  PW redundancy in this scenario requires that the
   forwarding status of the ACs at PE1 and PE3 be signaled to PE2 so
   that PE2 can choose which PW to make active.

   Changes occurring on the dual-homed side of the network due to a
   failure of the AC or PE are not propagated to the ACs on the other



Muley, et al.           Expires November 4, 2012                [Page 6]


Internet-Draft                PW Redundancy                     May 2012


   side of the network.  Furthermore, failures in the PSN are not
   propagated to the attached CEs.

3.2.2.  Multiple Multi-Homed CEs

   This scenario, illustrated in Figure 3, is also designed to protect
   the emulated service against failures of the ACs and failures of the
   PEs.  Both CE1 and CE2, are dual-homed to their respective PEs, PE1
   and PE2, and PE3 and PE4.  The method used by the CEs to choose which
   AC to use to forward traffic towards the PSN is determined by a dual-
   homing control protocol.  The details of this protocol are outside
   the scope of this document.

   Note that the PSN tunnels are not shown in this figure for clarity.
   However, it can be assumed that each of the PWs shown is encapsulated
   in a separate PSN tunnel.  Protection against failures of the PSN
   tunnels is provided using PSN mechanisms such as MPLS fast reroute,
   so that these failures do not impact the PW.
         |<-------------- Emulated Service ---------------->|
         |                                                  |
         |          |<------- Pseudowire ------->|          |
         |          |                            |          |
         |          |    |<-- PSN Tunnels-->|    |          |
         |          V    V                  V    V          |
         V    AC    +----+                  +----+     AC   V
   +-----+    |     |....|.......PW1........|....|     |    +-----+
   |     |----------| PE1|......   .........| PE3|----------|     |
   | CE1 |          +----+      \ /  PW3    +----+          | CE2 |
   |     |          +----+       X          +----+          |     |
   |     |          |    |....../ \..PW4....|    |          |     |
   |     |----------| PE2|                  | PE4|--------- |     |
   +-----+    |     |....|.....PW2..........|....|     |    +-----+
              AC    +----+                  +----+     AC

           Figure 3: Multiple Multi-Homed CEs with PW Redundancy

   PW1 and PW4 connect PE1 to PE3 and PE4, respectively.  Similarly, PW2
   and PW3 connect PE2 to PE4 and PE3.  PW1, PW2, PW3 and PW4 are all
   UP.  In order to support N:1 or 1:1 protection, only one PW MUST be
   selected to forward traffic.  This document defines an additional PW
   state that reflects this forwarding state, which is separate from the
   operational status of the PW.  This is the 'Preferential Forwarding
   Status'.

   If a PW has a preferential forwarding status of 'active', it can be
   used for forwarding traffic.  The actual UP PW chosen by the combined
   set of PEs connected to the CEs is determined by considering the
   preferential forwarding status of each PW at each PE.  The mechanisms



Muley, et al.           Expires November 4, 2012                [Page 7]


Internet-Draft                PW Redundancy                     May 2012


   for communicating the preferential forwarding status are outside the
   scope of this document.  Only one PW is used for forwarding.

   The following failure scenario illustrates the operation of PW
   redundancy in Figure 3.  In the initial steady state, when there are
   no failures of the ACs, one of the PWs is chosen as the active PW,
   and all others are chosen as standby.  The dual-homing protocol
   between CE1 and PE1/PE2 chooses to use the AC to PE2, while the
   protocol between CE2 and PE3/PE4 chooses to use the AC to PE4.
   Therefore the PW between PE2 and PE4 is chosen as the active PW to
   complete the path between CE1 and CE2.

   On failure of the AC between the dual-homed CE1 and PE2, the
   preferential forwarding status of the PWs at PE1, PE2, PE3 and PE4
   needs to change so as to re-establish a path from CE1 to CE2.
   Different mechanisms can be used to achieve this and these are beyond
   the scope of this document.  After the change in status, the
   algorithm needs to revaluate and select which PW to forward traffic
   on.  In this application, each dual-homing algorithm, i.e., {CE1,
   PE1, PE2} and {CE2, PE3, PE4}, selects the active AC independently.
   There is therefore a need to signal the active status of each AC such
   that the PEs can select a common active PW for forwarding between CE1
   and CE2.

   Changes occurring on one side of network due to a failure of the AC
   or PE are not propagated to the ACs on the other side of the network.
   Furthermore, failures in the PSN are not propagated to the attached
   CEs.  Note that end-to-end native service protection switching can
   also be used to protect the emulated service in this scenario.  In
   this case, PW3 and PW4 are not necessary.

   If the CEs do not perform native service protection switching, they
   may instead use load balancing across the paths between the CEs.

3.2.3.  Single-Homed CE With MS-PW Redundancy

   This application is shown in Figure 4.  The main objective is to
   protect the emulated service against failures of the S-PEs.













Muley, et al.           Expires November 4, 2012                [Page 8]


Internet-Draft                PW Redundancy                     May 2012


       Native   |<----------- Pseudowires ----------->|  Native
       Service  |                                     |  Service
        (AC)    |     |<-PSN1-->|     |<-PSN2-->|     |  (AC)
          |     V     V         V     V         V     V   |
          |     +-----+         +-----+         +-----+   |
   +----+ |     |T-PE1|=========|S-PE1|=========|T-PE2|   |   +----+
   |    |-------|......PW1-Seg1.......|.PW1-Seg2......|-------|    |
   | CE1|       |     |=========|     |=========|     |       | CE2|
   |    |       +-----+         +-----+         +-----+       |    |
   +----+        |.||.|                          |.||.|       +----+
                 |.||.|         +-----+          |.||.|
                 |.||.|=========|     |========== .||.|
                 |.||...PW2-Seg1......|.PW2-Seg2...||.|
                 |.| ===========|S-PE2|============ |.|
                 |.|            +-----+             |.|
                 |.|============+-----+============= .|
                 |.....PW3-Seg1.|     | PW3-Seg2......|
                  ==============|S-PE3|===============
                                |     |
                                +-----+

              Figure 4: Single-Homed CE with MS-PW Redundancy

   CE1 is connected to PE1 and CE2 is connected to PE2.  There are three
   multi-segment PWs.  PW1 is switched at S-PE1, PW2 is switched at
   S-PE2, and PW3 is switched at S-PE3.

   Since there is no multi-homing running on the ACs, the T-PE nodes
   advertise 'active' for the preferential forwarding status based on a
   priority for the PW.  The priority associates a meaning of 'primary
   PW' and 'secondary PW' to a PW.  These priorities MUST be used if
   revertive mode is used and the active PW to use for forwarding
   determined accordingly.  The priority can be derived via
   configuration or based on the value of the PW FEC.  For example, a
   lower value of PWid FEC can be taken as a higher priority.  However,
   this does not guarantee selection of same PW by the T-PEs because of,
   for example, a mismatch in the configuration of the PW priority at
   each T-PE.  The intent of this application is for T-PE1 and T-PE2 to
   synchronize the transmit and receive paths of the PW over the
   network.  In other words, both T-PE nodes are required to transmit
   over the PW segment which is switched by the same S-PE.  This is
   desirable for ease of operation and troubleshooting.

3.2.4.  PW Redundancy Between MTU-s in H-VPLS

   The following figure (based on the architecture shown in Figure 3 of
   [RFC4762]) illustrates the application of PW redundancy to
   hierarchical VPLS (H-VPLS).  Note that the PSN tunnels are not shown



Muley, et al.           Expires November 4, 2012                [Page 9]


Internet-Draft                PW Redundancy                     May 2012


   for clarity, and only one PW of a PW group is shown.  Here, a multi-
   tenant unit switch (MTU-s) is dual-homed to two PE router switches
   (PE-rs).


                                         PE1-rs
                                       +--------+
                                       |  VSI   |
                       Active PW       |   --   |
                        Group..........|../  \..|.
       CE-1                 .          |  \  /  | .
        \                  .           |   --   |  .
         \                .            +--------+   .
          \   MTU-s      .                  .        .     PE3-rs
           +--------+   .                   .         . +--------+
           |   VSI  |  .                    .  H-VPlS  .|  VSI   |
           |   -- ..|..                     .   Core    |.. --   |
           |  /  \  |                       .    PWs    |  /  \  |
           |  \  /..|..                     .           |  \  /  |
           |   --   |  .                    .          .|.. --   |
           +--------+   .                   .         . +--------+
          /              .                  .        .
         /                .            +--------+   .
        /                  .           |  VSI   |  .
       CE-2                 .          |   --   | .
                             ..........|../  \..|.
                       Standby PW      |  \  /  |
                        Group          |   --   |
                                       +--------+
                                         PE2-rs


                Figure 5: MTU-s Dual Homing in H-VPLS Core

   In Figure 5, the MTU-s is dual homed to PE1-rs and PE2-rs and has
   spoke PWs to each of them.  The MTU-s needs to choose only one of the
   spoke PWs (the active PW) to forward traffic to one of the PEs, and
   sets the other PW to standby.  The MTU-s can derive the status of the
   PWs based on local policy configuration.  PE1-rs and PE2-rs are
   connected to the H-VPLS core on the other side of network.  The MTU-s
   communicates the status of its member PWs for a set of virtual
   switching instances (VSIs) that share a common status of active or
   standby.  Here, the MTU-s controls the selection of PWs used to
   forward traffic.  Signaling using PW grouping with a common group-id
   in the PWid FEC Element, or a Grouping TLV in Generalized PWid FEC
   Element as defined in [RFC4447], to PE1-rs and PE2-rs, is recommended
   for improved scaling.




Muley, et al.           Expires November 4, 2012               [Page 10]


Internet-Draft                PW Redundancy                     May 2012


   Whenever an MTU-s performs a switchover of the active PW group, it
   needs to communicate this status change the PE2-rs.  That is, it
   informs PE2-rs that the status of the standby PW group has changed to
   active.

   In this scenario, PE devices are aware of switchovers at the MTU-s
   and could generate MAC Withdraw messages to trigger MAC flushing
   within the H-VPLS full-mesh.  By default, MTU-s devices should still
   trigger MAC withdraw messages as defined in [RFC4762] to prevent two
   copies of MAC withdraws to be sent (one by the MTU-s and another one
   by the PE-rs').  Mechanisms to disable the MAC withdraw trigger in
   certain devices are out of the scope of this document.

3.2.5.  PW Redundancy Between VPLS Network Facing PEs (n-PEs)

   The following figure illustrates the use of PW redundancy for dual-
   homed connectivity between PEs in a ring topology.  As above, PSN
   tunnels are not shown and only one PW of a PW group is shown for
   clarity.


               PE1                            PE2
            +--------+                     +--------+
            |  VSI   |                     |  VSI   |
            |   --   |                     |   --   |
      ......|../  \..|.....................|../  \..|.......
            |  \  /  |     PW Group 1      |  \  /  |
            |   --   |                     |   --   |
            +--------+                     +--------+
                 .                              .
                 .                              .
   VPLS Domain A .                              . VPLS Domain B
                 .                              .
                 .                              .
                 .                              .
            +--------+                     +--------+
            |  VSI   |                     |  VSI   |
            |   --   |                     |   --   |
      ......|../  \..|.....................|../  \..|........
            |  \  /  |     PW Group 2      |  \  /  |
            |   --   |                     |   --   |
            +--------+                     +--------+
               PE3                            PE4



                  Figure 6: Redundancy in a Ring Topology




Muley, et al.           Expires November 4, 2012               [Page 11]


Internet-Draft                PW Redundancy                     May 2012


   In Figure 6, PE1 and PE3 from VPLS domain A are connected to PE2 and
   PE4 in VPLS domain B via PW group 1 and PW group 2.  The PEs are
   connected to each other in such a way as to form a ring topology.
   Such scenarios may arise in inter-domain H-VPLS deployments where
   rapid spanning tree (RSTP) or other mechanisms may be used to
   maintain loop free connectivity of the PW groups.

   [RFC4762] outlines multi-domain VPLS services without specifying how
   multiple redundant border PEs per domain and per VPLS instance can be
   supported.  In the example above, PW group 1 may be blocked at PE1 by
   RSTP and it is desirable to block the group at PE2 by exchanging the
   PW preferential forwarding status of standby.  The details of how PW
   grouping is achieved and used is deployment specific and is outside
   the scope of this document.

3.2.6.  Redundancy in a VPLS Bridge Module Model

                          |<----- Provider ----->|
                                    Core
                   +------+                      +------+
                   | n-PE |::::::::::::::::::::::| n-PE |
        Provider   | (P)  |..........   .........| (P)  |  Provider
        Access     +------+          . .         +------+  Access
        Network                       X                    Network
          (1)      +------+          . .         +------+    (2)
                   | n-PE |..........   .........| n-PE |
                   |  (B) |......................| (B)  |
                   +------+                      +------+



                       Figure 7: Bridge Module Model



                            Bridge Module Model

   Figure 7 shows a scenario with two provider access networks.  Each
   network has two n-Pes.  These n-PEs are connected via a full mesh of
   PWs for a given VPLS instance.  As shown in the figure, only one n-PE
   in each access network serves as the primary PE (P) for that VPLS
   instance and the other n-PE serves as the backup PE (B).  In this
   figure, each primary PE has two active PWs originating from it.
   Therefore, when a multicast, broadcast, or unknown unicast frame
   arrives at the primary n-PE from the access network side, the n-PE
   replicates the frame over both PWs in the core even though it only
   needs to send the frames over a single PW (shown with :::: in the
   figure) to the primary n-PE on the other side.  This is an



Muley, et al.           Expires November 4, 2012               [Page 12]


Internet-Draft                PW Redundancy                     May 2012


   unnecessary replication of the customer frames that consumes core-
   network bandwidth (half of the frames get discarded at the receiving
   n-PE).  This issue gets aggravated when there is three or more n-PEs
   per provider, access network.  For example if there are three n-PEs
   or four n-PEs per access network, then 67% or 75% of core
   bandwidthfor multicast, broadcast and unknown unicast are wasted,
   respectively.

   In this scenario, the n-PEs can communicate the active or standby
   status of the PWs among them.  This status can be derived from the
   active or backup state of an n-PE for a given VPLS.


4.  Generic PW Redundancy Requirements

4.1.  Protection Switching Requirements

   o  Protection architectures such as N:1,1:1 or 1+1 are possible. 1:1
      protection MUST be supported.  The N:1 protection case is less
      efficient in terms of the resources that must be allocated and
      hence this SHOULD be supported. 1+1 protection MAY be used in the
      scenarios described in the document.  However, the details of its
      usage are outside the scope of this document.

   o  Non-revertive behavior MUST be supported, while revertive behavior
      is OPTIONAL.  This avoids the need to designate one PW as primary
      unless revertive behavior is explicitly required.

   o  Protection switchover can be initiated from a PE e.g. using a
      manual lockout/force switchover, or it may be triggered by a
      signal failure i.e. a defect in the PW or PSN.  Manual switchover
      may be necessary if it is required to disable one PW in a
      redundant set.  Both methods MUST be supported and signal failure
      triggers MUST be treated with a higher priority than any local or
      far-end manual trigger.

   o  Note that a PE MAY be able to forward packets received from a PW
      with a standby status in order to avoid black holing of in-flight
      packets during switchover.  However, in the case of use of VPLS,
      all VPLS application packets received from standby PWs MUST be
      dropped, except for OAM packets.

4.2.  Operational Requirements

   o  (T-)PEs involved in protecting a PW SHOULD automatically discover
      and attempt to resolve inconsistencies in the configuration of
      primary/secondary PW.




Muley, et al.           Expires November 4, 2012               [Page 13]


Internet-Draft                PW Redundancy                     May 2012


   o  (T-)PEs involved in protecting a PW SHOULD automatically discover
      and attempt to resolve inconsistencies in the configuration of
      revertive/non-revertive protection switching mode.

   o  (T-)PEs that do not automatically discover or resolve
      inconsistencies in the configuration of primary/secondary,
      revertive/non-revertive, or other parameters MUST generate an
      alarm upon detection of an inconsistent configuration.

   o  (T-)PEs participating in PW redundancy MUST support the
      configuration of revertive or non-revertive protection switching
      modes if both modes are supported.

   o  (T-)PEs participating in PW redundancy SHOULD support the local
      invocation of protection switching.

   o  (T-)PEs participating in PW redundancy SHOULD support the local
      invocation of a lockout of protection switching.


5.  Security Considerations

   This document requires extensions to the Label Distribution Protocol
   (LDP) that are needed for protecting pseudowires.  These will inherit
   at least the same security properties as LDP [RFC5036] and the PW
   control protocol [RFC4447].


6.  IANA Considerations

   This document has no actions for IANA.


7.  Major Contributing Authors

   The editors would like to thank Pranjal Kumar Dutta, Marc Lasserre,
   Jonathan Newton, Hamid Ould-Brahim, Olen Stokes, Dave Mcdysan, Giles
   Heron and Thomas Nadeau who made a major contribution to the
   development of this document.












Muley, et al.           Expires November 4, 2012               [Page 14]


Internet-Draft                PW Redundancy                     May 2012


   Pranjal Dutta
   Alcatel-Lucent
   Email: pranjal.dutta@alcatel-lucent.com

   Marc Lasserre
   Alcatel-Lucent
   Email: marc.lasserre@alcatel-lucent.com

   Jonathan Newton
   Cable & Wireless
   Email: Jonathan.Newton@cw.com

   Olen Stokes
   Extreme Networks
   Email: ostokes@extremenetworks.com

   Hamid Ould-Brahim
   Nortel
   Email: hbrahim@nortel.com

   Dave McDysan
   Verizon
   Email: dave.mcdysan@verizon.com

   Giles Heron
   Cisco Systems
   Email: giles.heron@gmail.com

   Thomas Nadeau
   Computer Associates
   Email: tnadeau@lucidvision.com


8.  Acknowledgements

   The authors would like to thank Vach Kompella, Kendall Harvey,
   Tiberiu Grigoriu, Neil Hart, Kajal Saha, Florin Balus and Philippe
   Niger for their valuable comments and suggestions.


9.  References

9.1.   Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-



Muley, et al.           Expires November 4, 2012               [Page 15]


Internet-Draft                PW Redundancy                     May 2012


              Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC4026]  Andersson, L. and T. Madsen, "Provider Provisioned Virtual
              Private Network (VPN) Terminology", RFC 4026, March 2005.

   [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
              Heron, "Pseudowire Setup and Maintenance Using the Label
              Distribution Protocol (LDP)", RFC 4447, April 2006.

   [RFC4762]  Lasserre, M. and V. Kompella, "Virtual Private LAN Service
              (VPLS) Using Label Distribution Protocol (LDP) Signaling",
              RFC 4762, January 2007.

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

   [RFC5659]  Bocci, M. and S. Bryant, "An Architecture for Multi-
              Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
              October 2009.

9.2.  Informative References

   [RFC6073]  Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
              Aissaoui, "Segmented Pseudowire", RFC 6073, January 2011.


Authors' Addresses

   Praveen Muley
   Alcatel-Lucent


   Email: praveen.muley@alcatel-lucent.com


   Mustapha Aissaoui
   Alcatel-Lucent


   Email: mustapha.aissaoui@alcatel-lucent.com


   Matthew Bocci
   Alcatel-Lucent


   Email: matthew.bocci@alcatel-lucent.com




Muley, et al.           Expires November 4, 2012               [Page 16]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/