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Versions: (draft-muley-dutta-pwe3-redundancy-bit) 00 01 02 03 04 05 06 07 08 09 RFC 6870

Network Working Group                                Praveen Muley, Ed.
Internet Draft                                   Mustapha Aissaoui, Ed.
Intended Status: Standards Track                         Alcatel-Lucent
Expires: March 22, 2012




                                                     September 22, 2011

               Pseudowire Preferential Forwarding Status Bit
                  draft-ietf-pwe3-redundancy-bit-05.txt


Abstract

   This document describes a mechanism for standby status signaling of
   redundant pseudowires (PWs) between their termination points. A set
   of redundant PWs is configured between provider edge (PE) nodes in
   single-segment pseudowire (SS-PW) applications, or between
   terminating provider edge (T-PE) nodes in multi-segment pseudowire
   (MS-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 status bit is needed
   to indicate a preferential forwarding status of Active or Standby for
   each PW in a redundant set.

   In addition, a second status bit is defined to allow peer PE nodes to
   coordinate a switchover operation of the PW.



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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 March 22, 2012.




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Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   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.



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 [1].



Table of Contents


   1. Introduction......................................... 3
   2. Motivation and Scope.................................. 5
   3. Terminology ......................................... 7
   4. PE Architecture...................................... 8
   5. Modes of Operation.................................... 9
      5.1. Independent Mode: ................................ 9
      5.2. Master/Slave Mode:............................... 11
   6. PW State Transition Signaling Procedures................. 12
      6.1. PW Standby Notification Procedures in Independent mode. 13
      6.2. PW Standby notification procedures in Master/Slave mode 13
         6.2.1. PW State Machine............................ 14
      6.3. Coordination of PW Switchover...................... 15
         6.3.1. Procedures at the requesting endpoint........... 17
         6.3.2. Procedures at the receiving endpoint............ 18
   7. Status Mapping...................................... 19
      7.1. AC Defect State Entry/Exit........................ 19
      7.2. PW Defect State Entry/Exit........................ 19


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   8. Applicability and Backward Compatibility................. 20
   9. Security Considerations............................... 20
   10. MIB Considerations.................................. 20
   11. IANA Considerations................................. 21
      11.1. Status Code for PW Preferential Forwarding Status.... 21
      11.2. Status Code for PW Request Switchover Status ........ 22
   12. Major Contributing Authors........................... 22
   13. Acknowledgments.................................... 23
   14. References ........................................ 23
      14.1. Normative References............................ 23
      14.2. Informative References .......................... 23
   15. Appendix A - Applications of PW Redundancy Procedures..... 24
      15.1. One Multi-homed CE with single SS-PW redundancy...... 24
      15.2. Multiple Multi-homed CEs with single SS-PW redundancy. 26
      15.3. Multi-homed CE with MS-PW redundancy............... 27
      15.4. Multi-homed CE with MS-PW redundancy and S-PE protection28
      15.5. Single Homed CE with MS-PW redundancy.............. 30
      15.6. PW redundancy between H-VPLS MTU-s and PE-rs ........ 32
   Author's Addresses..................................... 33

1. Introduction

   In Virtual Private Wire Services (VPWS) or Virtual Private Local Area
   network Services (VPLS) that use SS-PWs, 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)
   label switched path (LSP) with a fast reroute (FRR) backup or an end-
   to-end backup LSP. There are, however, applications where PSN
   protection is insufficient to fully protect the PW-based service.
   These include the following:

   In a VPWS service where the Customer Edge (CE) node is dual homed to
   a pair of PE nodes, PW redundancy mechanisms are required to ensure
   that the correct PW is used for forwarding when attachment circuit
   (AC) redundancy is used. PW redundancy mechanisms are also required
   when multiple redundant MS-PWs are used between T-PEs, to ensure that
   both T-PEs use the same MS-PW to forward to one another.

   In a hierarchical VPLS (H-VPLS) service, PW redundancy mechanisms are
   required to enable a multi-tenant unit switch (MTU-s) to be dual-
   homed to two PE-rs devices.

   In these cases, pseudowire redundancy mechanisms are required. These
   scenarios are described in the PW redundancy and framework document
   [5].




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   Scenarios, such as those above, therefore rely on a set of two or
   more pseudowires to protect a given VPWS or VPLS . Only one of these
   pseudowires is used by the PEs to forward user traffic on at any
   given time. This is the active PW. The other PWs in the set are
   considered standby and are not used for forwarding unless they become
   active. This provides a 1:1 or N:1 PW protection with the possibility
   of multi-homing between the CE and the PEs.

   In order to support AC or spoke PW redundancy, at least one of the
   PEs on which a PW terminates must be different from that on which the
   primary PW terminates, as described in [5]. Figure 1-1 illustrates an
   application of active and standby PWs.





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


                 Figure 1-1: Reference Model for PW Redundancy

   In MS-PW applications, PW redundancy is also required to protect the
   service against failures of the switching PEs, which cannot be
   protected by PSN mechanisms. In addition, PW redundancy is also
   required if CEs are dual-homed to the PEs, as described above. In
   this case, multiple MS-PWs are configured between a pair of T-PE
   nodes, as described in Figure 2 of [5]. The paths of these MS-PWs are
   diverse in that they are switched at different S-PE nodes. Only one
   of these MS-PWs is active at any given time, while the others are
   standby.




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   This document specifies a new PW status bit to indicate the
   preferential forwarding status of the PW for the purpose of notifying
   the remote PE of the preferential forwarding state of each PW in the
   redundancy set i.e. Active or Standby. This status bit is different
   from the PW status bits already defined in the PWE3 control protocol
   [2]. In addition, a second status bit is defined to allow peer PE
   nodes to coordinate a switchover operation of the PW from Active to
   standby, or vice versa.

2. Motivation and Scope

   The PWE3 control protocol [2] defines the following status codes in
   the PW status TLV to indicate the state for an AC and a PW:

   0x00000000 - Pseudowire forwarding (clear all failures)

   0x00000001 - Pseudowire Not Forwarding

   0x00000002 - Local Attachment Circuit (ingress) Receive Fault

   0x00000004 - Local Attachment Circuit (egress) Transmit Fault

   0x00000008 - Local PSN-facing PW (ingress) Receive Fault

   0x00000010 - Local PSN-facing PW (egress) Transmit Fault

   The scenarios defined in [5] allow the provisioning of a primary PW
   and one or many secondary PWs in the same VPWS or VPLS service.

   A PE node makes a selection of which PW to activate at any given time
   for the purpose of forwarding user packets. This selection takes into
   account the local state of the PW and AC, as well as the remote state
   of the PW and AC as indicated in the PW status bits it received from
   the peer PE node.

   In the absence of faults, all PWs are UP both locally and remotely
   and a PE node needs to select a single PW to forward user packets to.
   This is referred to as the active PW. All other PWs will be in
   standby and must not be used to forward user packets.

   In order for both ends of the service to select the same PW for
   forwarding user packets, this document defines a new status bit, the
   'Preferential Forwarding' status bit, to allow a PE node to indicate
   the preferential forwarding state of a PW to its peer PE node.





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   In addition, a second status bit is defined to allow peer PE nodes to
   coordinate a switchover operation of the PW if required by the
   application. This is known as the 'request switchover' status bit.

   Together, the mechanisms described in this document achieve the
   following PW protection capabilities:

      a. A MANDATORY 1:1 PW protection with a single active PW and one
         standby PW. An active PW can forward data traffic and control
         plane traffic, such as OAM packets. A standby PW does not
         carry data traffic.

      b. An OPTIONAL N:1 PW protection scheme with a single active PW
         and N standby PWs.

      c. An OPTIONAL mechanism to allow PW endpoints to coordinate the
         switchover to a given PW by using an explicit
         request/acknowledgment switchover procedure. This mechanism is
         complementary to the Independent mode of operation and is
         described in Section 6.3. . This mechanism can be invoked
         manually by the user, effectively providing a manual
         switchover capability. It can also be invoked automatically to
         resolve a situation where the PW endpoints could not match the
         two directions of the PW.

      d. An OPTIONAL, locally configured precedence to govern the
         selection of a PW when more than one PW qualify for the active
         state, as defined in sections 5.1. and 5.2. The PW with the
         lowest precedence value has the highest priority. Precedence
         may be configured via, for example, a local configuration
         parameter at the PW endpoint.

      e. OPTIONALLY, implementations can designate by configuration one
         PW in the 1:1 or N:1 set as a primary PW and the remaining as
         secondary PWs. If more than one PW qualify for the active
         state, as defined in sections 5.1. and 5.2. , a PE node
         selects the primary PW in preference to a secondary PW. In
         other words, the primary PW has implicitly the lowest
         precedence value. Furthermore, a PE node reverts to the
         primary PW immediately after it comes back up or after the
         expiration of a delay. The PE node can use the PW precedence
         to select a secondary PW among many that qualify for active
         state.

   These protection schemes are provided using the following operational
   modes:



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          1. An independent mode of operation in which each PW endpoint
             node uses its own local rule to select which PW it intends
             to activate at any given time and advertises it to the
             remote endpoints. Only a PW which is UP and which indicated
             Active status bit locally and remotely is in the active
             state and can be used to forward data packets. This is
             described in Section 5.1.

          2. A Master/Slave mode in which one PW endpoint, the Master
             endpoint, selects and dictates to the other endpoint(s),
             the Slave endpoint(s), which PW to activate. This is
             described in Section 5.2.

   The above mechanisms and operational modes allow the following:

          a.Multi-homing of a CE device to two or more PE nodes.

          b.Multi-homing of a PE node to two or more PE nodes.

   More details of how these schemes are used can be found in
   Informative Appendix A.

   Note that this document specifies the mechanisms to support PW
   redundancy where a set of redundant PWs terminate on either a PE (for
   SS-PW) or a T-PE (for MS-PW). PW redundancy scenarios where the
   redundant set of PW segments terminate on an S-PE are for further
   study.

3. Terminology

   UP PW:   A PW which has been configured (label mapping exchanged
            between PEs) and is not in any of the PW or AC defect
            states specified in [2]. Such a PW is available for
            forwarding traffic.

   DOWN PW: A PW that has either not been fully configured, or has been
            configured and is in any of the PW or AC defect states
            specified in [2], such a PW is not available for forwarding
            traffic.

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

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




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   Primary PW: The PW which a PW endpoint activates in preference to any
           other PW when more than one PW qualifies for active state.
           When the primary PW comes back up after a failure and
           qualifies for active state, the PW endpoint always reverts
           to it. The designation of Primary is performed by local
           configuration for the PW at the PE.

   Secondary PW: When it qualifies for 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.

   PW Precedence: This is a configuration local to the PE that dictates
           the order in which a forwarder chooses to use a PW when
           multiple PWs all qualify for the active state. Note that a
           PW which has been configured as Primary has implicitly the
           lowest precedence value.

   PW Endpoint: A PE where a PW terminates on a point where Native
           Service Processing is performed, e.g., A SS-PW PE, an MS-PW
           T-PE, or an H-VPLS MTU-s or PE-rs.

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

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

4. PE Architecture

   Figure 4-1 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 [6]. The
   forwarder selects which of the redundant PWs to using the criteria
   described in this document.












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              +----------------------------------------+
              |                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 4-1 PE Architecure for PW redundancy


5. Modes of Operation

   There are two modes of operation for the use of the PW Preferential
   Forwarding status bits:

   o  Independent mode

   o  Master/Slave mode.

5.1. Independent Mode:

   PW endpoint nodes independently select which PW they intend to make
   active and which PWs they intend to make standby. They advertise the
   corresponding Active/Standby preferential forwarding status for each
   PW. Each PW endpoint compares local and remote status bits and uses
   the PW that is UP at both endpoints and that advertised Active
   preferential forwarding status at both the local and remote
   endpoints.

   In this mode of operation, the preferential forwarding status
   indicates the preferred forwarding state of each endpoint but the
   actual forwarding state of the PW is the result of the comparison of
   the local and remote forwarding status bits.

   If more than one PW qualifies for the Active state, each PW endpoint
   MUST implement a common mechanism to choose the PW for forwarding. If
   this mechanism uses a precedence value for the PW, it must use the PW



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   with the lowest configured precedence value. The precedence parameter
   is optional.

   If more than one PW qualify for the Active state, and the PW endpoint
   is configured with one PW as primary, it must use the primary PW in
   preference to all secondary PWs. If a primary PW is not available, it
   must use the secondary PW with the lowest precedence value. If the
   primary PW becomes available, a PW endpoint must revert to it
   immediately or after the expiration of a configurable delay. These
   primary/secondary procedures are optional.

   In steady state with consistent configuration, a PE will always find
   an active PW. However, it is possible that such a PW is not found due
   to a misconfiguration. In the event that an active PW is not found, a
   management indication SHOULD be generated. If a management indication
   for failure to find an active PW was generated and an active PW is
   subsequently found, a management indication should be generated, so
   clearing the previous failure indication. Additionally, a PE may use
   the optional request switchover procedures described in Section 6.3.
   to have both PE nodes switch to a common PW.

   There may also be transient conditions where endpoints do not share a
   common view of the Active/Standby state of the PWs. This could be
   caused by propagation delay of the T-LDP status messages between
   endpoints. In this case, the behavior of the receiving endpoint is
   outside the scope of this document.

   Thus, in this mode of operation, the following definition of Active
   and Standby PW states apply:

   o  Active State

   A PW is considered to be in Active state when the PW labels are
   exchanged between its two endpoints and the status bits exchanged
   between the endpoints indicate the PW is UP and its preferential
   forwarding status is Active at both endpoints. In this state user
   traffic can flow over the PW in both directions. As described in
   Section 5.1. , the PE nodes must implement a common mechanism to
   select one PW for forwarding in case multiple PWs qualify for the
   Active state.

   o  Standby State

   A PW is considered to be in Standby state when the PW labels are
   exchanged between its two endpoints, but the Preferential Forwarding
   status bits exchanged indicate the PW preferential forwarding status
   is Standby at one or both endpoints. In this state the endpoints MUST


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   NOT forward data traffic over the PW but MAY allow PW OAM packets,
   e.g., VCCV, to be sent and received in order to test the liveliness
   of standby PWs. The endpoints of the PW may also allow the forwarding
   of specific control plane packets of applications using the PW. The
   specification of applications and the allowed control plane packets
   is outside the scope of this document. If the PW is a spoke in H-
   VPLS, any MAC addresses learned via the PW SHOULD be flushed when it
   transitions to Standby state according to the procedures in RFC4762
   [3] and [9].

5.2. Master/Slave Mode:

   One endpoint node of the redundant set of PWs is designated the
   Master and is responsible for selecting which PW both endpoints must
   use to forward user traffic.

   The Master indicates the forwarding state in the PW Preferential
   Forwarding status bit. The other endpoint node, the Slave, MUST
   follow the decision of the Master node based on the received status
   bits. In other words, the Preferential Forwarding status bit sent by
   the Master node indicates the actual forwarding state of the PW at
   the Master node.

   One endpoint of the PW, the Master, actively selects which PW to
   activate and uses it for forwarding user traffic. This status is
   indicated to the Slave node by setting the Preferential Forwarding
   status bit in the status bit TLV to Active. It does not forward user
   traffic to any other of the PW's in the redundancy set to the slave
   node and indicates this by setting the Preferential Forwarding status
   bit in the status bit TLV to Standby for those PWs. The master node
   MUST ignore any PW Preferential Forwarding status bits received from
   the Slave nodes.

   If more than one PW qualify for the Active state, and the PW endpoint
   is configured with one PW as primary, the Master endpoint must use
   the primary PW in preference to all secondary PWs. If a primary PW is
   not available, it must use the secondary PW with the lowest
   precedence value. If the primary PW becomes available, a PW endpoint
   must revert to it immediately or after the expiration of a
   configurable delay. These primary/secondary procedures are optional.

   The Slave endpoint(s) are required to act on the status bits received
   from the Master. When the received status bit transitions from Active
   to Standby, a Slave node MUST stop forwarding over the previously
   active PW. When the received status bit transitions from Standby to
   Active for a given PW, the Slave node MUST start forwarding user
   traffic over this PW.


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   There is a single PE Master PW endpoint node and one or many PE PW
   endpoint Slave nodes. The assignment of Master/Slave roles to the PW
   endpoints is performed by local configuration. Note that the above
   behavior assumes correct configuration of the Master and Slave
   endpoints. This document does not define a mechanism to detect errors
   in the configuration.

   In this mode of operation, the following definition of Active and
   Standby PW states apply:

   o  Active State

   A PW is considered to be in Active state when the PW labels are
   exchanged between its two endpoints, and the status bits exchanged
   between the endpoints indicate the PW is UP at both endpoints, and
   the preferential forwarding status at the Master endpoint is Active.
   In this state user traffic can flow over the PW in both directions.

   o  Standby State

   A PW is considered to be in Standby state when the PW labels are
   exchanged between its two endpoints, and the status bits exchanged
   between the endpoints indicate the preferential forwarding status at
   the Master endpoint is Standby. In this state the endpoints MUST NOT
   forward data traffic over the PW but MAY allow PW OAM packets, e.g.,
   VCCV, to be sent and received. The endpoints of the PW may also allow
   the forwarding of specific control plane packets of applications
   using the PW. The specification of applications and the allowed
   control plane packets is outside the scope of this document. If the
   PW is a spoke in H-VPLS, any MAC addresses learned via the PW SHOULD
   be flushed when it transitions to standby state according to the
   procedures in RFC4762 [3] and [9].

6. PW State Transition Signaling Procedures

   This section describes the extensions to PW status signaling and the
   processing rules for these extensions. It defines a new "PW
   Preferential Forwarding" bit Status Code that is to be used with the
   PW Status TLV specified in RFC 4447 [2].

   The PW Preferential Forwarding bit, when set, is used to signal
   either the Preferred or Actual Active/Standby forwarding state of the
   PW by one PE to the far end PE. The actual semantics of the value
   being signaled vary according to whether the PW is acting in a
   Master/Slave or Independent mode.




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6.1. PW Standby Notification Procedures in Independent mode

   PEs that contain PW endpoints independently select which PW they
   intend to use for forwarding, depending on the specific application
   (example applications are described in [5]). They advertise the
   corresponding preferred Active/Standby forwarding state for each PW.
   An Active Preferential Forwarding state is indicated by clearing the
   PW Preferential Forwarding status bit in the PW status TLV. A Standby
   Preferential Forwarding State is indicated by setting the PW
   Preferential Forwarding status bit in the PW status TLV. This
   advertisement occurs in both the initial label mapping message and in
   a subsequent notification message when the forwarding state
   transitions as a result of a state change in the specific
   application.

   Each PW endpoint compares the updated local and remote status and
   effectively activates the PW which is UP at both endpoints and which
   shows both local Active and remote Active Preferential Forwarding
   states. The PE nodes must implement a common mechanism to select one
   PW for forwarding in case multiple PWs qualify for the Active state.

   When a PW is in Active state, the PEs can forward user packets, OAM
   packets, and other control plane packets over the PW.

   When a PW is in Standby state, the PEs MUST NOT forward user packets
   over the PW but MAY forward PW OAM packets and specific control plane
   packets.

   For MS-PWs, S-PEs MUST relay the PW status notification containing
   both the existing status bits and the new Preferential Forwarding
   status bits between ingress and egress PWs as per the procedures
   defined in [4].

6.2. PW Standby notification procedures in Master/Slave mode

   Whenever the Master PW endpoint selects or deselects a PW for
   forwarding user traffic at its end, it explicitly notifies the event
   to the remote Slave endpoint.  The slave endpoint carries out the
   corresponding action on receiving the PW state change notification.

   If the PW Preferential Forwarding bit in PW Status TLV received by
   the slave is set, it indicates that the PW at the Master end is not
   used for forwarding and is thus kept in the Standby state, the PW
   MUST also not be used for forwarding at Slave endpoint. Clearing the
   PW Preferential Forwarding bit in PW Status TLV indicates that the PW
   at the Master endpoint is used for forwarding and is in Active state,



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   and the receiving Slave endpoint MUST activate the PW if it was
   previously not used for forwarding.

   When this mechanism is used, a common Group ID in the PWid FEC
   element or a PW Grouping TLV in the Generalized PWid FEC Element as
   defined in [2] MAY be used to signal PWs in groups in order to
   minimize the number of LDP status messages that must be sent. When
   PWs are provisioned with such grouping a termination point sends a
   single "wildcard" Notification message to denote this change in
   status for all affected PWs. This status message contains either the
   PW FEC TLV with only the Group ID, or else it contains the PW
   Generalized FEC TLV with only the PW Grouping ID TLV. As mentioned in
   [2], the Group ID field of the PWid FEC Element, or the PW Grouping
   TLV in the Generalized PWid FEC Element, can be used to send status
   notification for an arbitrary set of PWs.

   For MS-PWs, S-PEs MUST relay the PW status notification containing
   both the existing and the new Preferential Forwarding status bits
   between ingress and egress PW segments as per the procedures defined
   in [4].

6.2.1. PW State Machine

   It is convenient to describe the PW state change behavior in terms of
   a state machine (Table 1). The PW state machine is explained in
   detail in the two defined states and the behavior is presented as a
   state transition table. The same state machine is applicable to PW
   Groups.





















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      STATE         EVENT                                   NEW STATE

      ACTIVE        PW put in Standby (master)              STANDBY
                    Action: Transmit PW preferential
                            forwarding bit set

                    Receive PW Preferential Forwarding      STANDBY
                       bit set   (slave)
                    Action: Stop forwarding over PW

                    Receive PW Preferential Forwarding      ACTIVE
                       bit set but bit not supported
                    Action: None

                    Receive PW Preferential Forwarding      ACTIVE
                       bit clear
                    Action: None.


      STANDBY       PW activated (master)                   ACTIVE
                    Action: Transmit PW preferential
                      forwarding bit clear

                    Receive PW Preferential Forwarding      ACTIVE
                       bit clear (slave)
                    Action: Activate PW

                    Receive PW Preferential Forwarding      STANDBY
                       bit clear but bit not supported
                    Action: None

                    Receive PW Preferential Forwarding      STANDBY
                       bit set
                    Action: No action


         Table 1  PW State Transition Table in Master/Slave Mode


6.3. Coordination of PW Switchover

   There are PW redundancy applications which require that PE nodes
   coordinate the switchover to a PW such that both endpoints will
   forward over the same PW at any given time. One such application for


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   redundant MS-PW is identified in [5]. Multiple MS-PWs are configured
   between a pair of T-PE nodes. The paths of these MS-PWs are diverse
   and are switched at different S-PE nodes. Only one of these MS-PWs is
   active at any given time. The others are put in standby. The
   endpoints follow the Independent Mode procedures to use the PW which
   is both UP and for which both endpoints advertise an Active
   'Preferential Forwarding' status bit.

   The trigger for sending a request to switchover of the MS-PW by one
   endpoint can be an operational event, for example a failure, which
   causes the endpoints to be unable to find a common PW for which both
   endpoints advertise an Active 'Preferential Forwarding' status bit.
   The other trigger is the execution of an administrative maintenance
   operation by the network operator in order to move the traffic away
   from the nodes or links currently used by the active PW.

   Unlike the case of a Master/Slave mode of operation, the endpoint
   requesting the switchover requires explicit acknowledgement from the
   peer endpoint that the request can be honored before it switches to
   another PW. Furthermore, any of the endpoints can make the request to
   switchover.

   This document specifies a second status bit that is used by a PE to
   request that its peer PE switchover to use a different active PW.
   This bit is referred to as the 'request PW switchover' status bit.
   The 'Preferential Forwarding' status bit continues to be used by each
   endpoint to indicate its current local settings of the Active/Standby
   state of each PW in the redundancy set. In other words, as in the
   Independent mode, it indicates to the far-end which of the PWs is
   being used to forward packets and which is being put in standby. It
   can thus be used as a way for the far-end to acknowledge the
   requested switchover operation.

   The request switchover bit is OPTIONAL and, if received by a PE, is
   ignored if not understood.

   If the request switchover bit is supported by both sending and
   receiving PEs, the following procedures MUST be followed by both
   endpoints of a PW to coordinate the switchover of the PW.

   S-PEs nodes MUST relay the PW status notification containing the
   existing status bits, as well as the new 'Preferential Forwarding'
   and 'request switchover' status bits between ingress and egress PW
   segments as per the procedures defined in [4].





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6.3.1. Procedures at the requesting endpoint

   a. The requesting endpoint sends a Status TLV in the LDP
      notification message with the 'request switchover' bit set on the
      PW it desires to switch to.

   b. The endpoint does not activate forwarding on that PW at this
      point in time. It MAY, however, enable receiving on that PW. Thus
      the 'Preferential Forwarding' status bit still reflects the
      currently-used PW.

   c. The requesting endpoint starts a timer while waiting the remote
      endpoint to acknowledge the request.

   d. If while waiting for the acknowledgment, the requesting endpoint
      receives a request from its peer to switchover to the same or a
      different PW, it must perform the following:

            i. If its address is higher than that of the peer, this
               endpoint ignores the request and continues to wait for
               the acknowledgement from its peer.

           ii. If its system IP address is lower than that of its peer,
               it aborts the timer and immediately starts the
               procedures of the receiving endpoint in Section 6.3.2.

   e.    If while waiting for the acknowledgment, the requesting
      endpoint receives a status notification message from its peer
      with the 'Preferential Forwarding' status bit cleared in the
      requested PW, it must treat this as an explicit acknowledgment of
      the request and must perform the following:

            i. Abort the timer.

           ii. Activate the PW.

          iii. Send an update status notification message with the
               'Preferential Forwarding' status bit and the 'request
               switchover' bit clear on the newly active PW and send an
               update status notification message with the
               'Preferential Forwarding' status bit set in the
               previously active PW.

   f. If while waiting for the acknowledgment, the requesting endpoint
      detects that the requested PW went into DOWN state locally, and
      could use an alternate PW which is UP, it must perform the
      following:


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            i. Abort the timer.

           ii. Issue a new request to switchover to the alternate PW.

          iii. Re-start the timer.

   g. If, while waiting for the acknowledgment, the requesting endpoint
      detects that the requested PW went into the DOWN state locally,
      and could not use an alternate PW which is UP, it must perform
      the following:

            i. Abort the timer.

           ii. Send an update status notification message with the
               'Preferential Forwarding' status bit unchanged and the
               'request switchover' bit reset for the requested PW.

   h. If, while waiting for the acknowledgment, the timer expires, the
      requesting endpoint MUST assume that the request was rejected and
      MAY issue a new request.

   i. If the requesting node receives the acknowledgment after the
      request expired, it will treat it as if the remote endpoint
      unilaterally switched between the PWs without issuing a request.
      In that case, it may issue a new request and follow the
      requesting endpoint procedures to synchronize which PW to use for
      the transmit and receive directions of the emulated service.

6.3.2. Procedures at the receiving endpoint

   a. Upon receiving a status notification message with the 'request
      switchover' bit set on a PW different from the currently active
      one, and the requested PW is UP, the receiving endpoint must
      perform the following:

           i. Activate the PW.

          ii. Send an update status notification message with the
               'Preferential Forwarding' status bit clear and the
               'request switchover' bit reset on the newly active PW ,
               and send an update status notification message with the
               'Preferential Forwarding' status bit set in the
               previously active PW.

         iii. Upon receiving a status notification message with the
               'request switchover' bit set on a PW different from the



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               currently active one, and the requested PW is DOWN, the
               receiving endpoint MUST ignore the request.

7. Status Mapping

   The generation and processing of the PW Status TLV must follow the
   procedures in RFC 4447 [2]. The PW status TLV is sent on the active
   PW and standby PWs to make sure the remote AC and PW states are
   always known to the local PE node.

   The generation and processing of PW Status TLV by an S-PE node in a
   MS-PW must follow the procedures in [4].

   The procedures for determining and mapping PW and AC states must
   follow the rules in [7] with the following modifications.

7.1. AC Defect State Entry/Exit

   A PE enters the AC receive (or transmit) defect state for a PW when
   one or more of the conditions specified for this PW type in [7] are
   met.

   When a PE enters the AC receive (or transmit) defect state for a PW,
   it must send a forward (reverse) defect indication to the remote
   peers over all PWs in the redundancy set when required by the PW type
   in [7].

   When a PE exits the AC receive (or transmit) defect state for a PW
   service, it must clear the forward (or reverse) defect indication to
   the remote peers over all PWs in the redundancy set when required by
   the PW type in [7].

7.2. PW Defect State Entry/Exit

   A PE enters the PW receive (or transmit) defect state for a PW
   service when one or more of the conditions specified in Section 8.2.1
   (Section 8.2.2) in [7] are met for all PWs in the redundancy set.

   When a PE enters the PW receive (or transmit) defect state for a PW
   service, it must send a reverse (or forward) defect indication over
   one or more of the PWs in the redundancy set if the PW failure was
   detected by this PE without receiving a forward defect indication
   from the remote PE [7].

   When a PE exits the PW receive (or transmit) defect state for a PW,
   it must clear the reverse (or forward) defect indication over any PW
   in the redundancy set if applicable.


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8. Applicability and Backward Compatibility

   The mechanisms defined in this document are applicable to
   applications where standby state signaling of a PW or PW group is
   required. Both PW FEC 128 and 129 are supported. All PWs which are
   part of a redundant set must use the same FEC type. When the set uses
   FEC 128 PWs, each PW is uniquely identified by its PW-ID. When the
   redundant set uses FEC 129 PWs, each PW must have a unique identifier
   which consists of the triplet AGI::SAII::TAII.

   A PE implementation that uses the mechanisms described in this
   document MUST negotiate the use of PW status TLV between its T-LDP
   peers as per RFC 4447 [2]. If PW Status TLV is found to be not
   supported by either of its endpoint after status negotiation
   procedures, then the mechanisms specified in this document cannot be
   used.

   A PE implementation compliant to RFC 4447 [2], and which does not
   support the generation or processing of the 'Preferential Forwarding'
   status bit or of the 'request switchover' status bit, will ignore
   these status bits if they are received from a peer PE.

9. Security Considerations

   This document uses the LDP extensions that are needed for protecting
   pseudowires. It will have the same security properties as in the PWE3
   control protocol [2].

10. MIB Considerations

   This document makes the following update to the PwOperStatusTC
   textual convention in RFC5542 [8]:

   PwOperStatusTC ::= TEXTUAL-CONVENTION
         STATUS      current
         DESCRIPTION
           "Indicates the operational status of the PW.



       - up(1):             Ready to pass packets.

       - down(2):           PW signaling is not yet finished, or
                            indications available at the service
                            level indicate that the PW is not
                            passing packets.



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       - testing(3):        AdminStatus at the PW level is set to
                            test.



       - dormant(4):   The PW is not in a condition to pass
                       packets but is in a 'pending' state,
                       waiting for some external event. For example, the
   PW Preferential Forwarding status state machine as defined in
   [RFCXXXX (this document)] is in state "STANDBY".

       - notPresent(5): Some component is missing to accomplish
                        the setup of the PW.  It can be
                        configuration error, incomplete
                        configuration, or a missing H/W component.

       - lowerLayerDown(6): One or more of the lower-layer interfaces
                            responsible for running the underlying PSN
                            is not in OperStatus 'up' state."

       SYNTAX   INTEGER {
           up(1),
           down(2),
           testing(3),
           dormant(4),
           notPresent(5),
           lowerLayerDown(6)
           }



11. IANA Considerations


   This document defines the following PW status codes for the PW
   redundancy application. IANA is requested to allocate these from the
   PW Status Codes registry.


11.1. Status Code for PW Preferential Forwarding Status


   0x00000020 When the bit is set, it indicates "PW forwarding

              standby".

              When the bit is cleared, it indicates "PW forwarding


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              active".

11.2. Status Code for PW Request Switchover Status


   0x00000040  When the bit is set, it represents "Request switchover to
               this PW".

               When the bit is cleared, it represents no specific
               action.

12. Major Contributing Authors

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

   Matthew Bocci
   Alcatel-Lucent
   Email: matthew.bocci@alcatel-lucent.com

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

   Giles Heron
   Cisco Systems, Inc.
   giles.heron@gmail.com

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

   Luca Martini
   Cisco Systems, Inc.
   Email: lmartini@cisco.com

   Thomas Nadeau
   Huawei
   tnadeau@lucidvision.com

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




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

   Olen Stokes
   Extreme Networks
   Email: ostokes@extremenetworks.com



13. Acknowledgments

   The authors would like to thank the following individuals for their
   valuable comments and suggestions which improved the document both
   technically and editorially:

   Vach Kompella, Kendall Harvey, Tiberiu Grigoriu, John Rigby,
   Prashanth Ishwar, Neil Hart, Kajal Saha, Florin Balus, Philippe
   Niger, Dave McDysan, Roman Krzanowski, Italo Busi, Robert Rennison,
   Nicolai Leymann and Daniel Cohn.

14. References

14.1. Normative References

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

   [2]   Martini, L., et al., "Pseudowire Setup and Maintenance using
         LDP", RFC 4447, April 2006.

   [3]   Kompella,V., Lasserrre, M. , et al., "Virtual Private LAN
         Service (VPLS) Using LDP Signalling", RFC 4762, January 2007.



14.2. Informative References

   [4]   Martini, L., et al., "Segmented Pseudo Wire", RFC 6073, January
         2011.

   [5]   Muley, P., et al., "Pseudowire (PW) Redundancy", draft-ietf-
         pwe3-redundancy-05.txt", September 2011.

   [6]   Bryant, S., et al., "Pseudo Wire Emulation Edge-to-Edge (PWE3)
         Architecture", RFC 3985, March 2005



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   [7]   Aissaoui, M., et al., "Pseudo Wire (PW) OAM Message Mapping",
         RFC 6310, July 2011.

   [8]   Nadeau, T., Zelig, D., Nicklass, O., "Definitions of Textual
         Conventions for Pseudowire (PW) Management", RFC5542, May 2009

   [9]   Dutta, P., Lasserre, M., Stokes, O., "LDP Extensions for
         Optimized MAC Address Withdrawal in H-VPLS", draft-ietf-l2vpn-
         vpls-ldp-mac-opt-04.txt, June 2011







15. Appendix A - Applications of PW Redundancy Procedures

   This section shows how the mechanisms described in this document are
   used to achieve the desired protection behavior for the scenarios
   described in the PW redundancy requirements and framework document
   [5].

15.1. One Multi-homed CE with single SS-PW redundancy

   The following figure illustrates an application of single segment
   pseudowire redundancy.

         |<-------------- 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 15-1 Multi-homed CE with single SS-PW redundancy



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   The application in Figure 15-1 makes use of the Independent mode of
   operation.

   CE1 is dual homed to PE1 and to PE3 by attachment circuits. The
   method for dual-homing of CE1 to PE1 and to PE3 nodes, and the
   protocols used, are outside the scope of this document (see [5]).

   In this example, the AC from CE1 to PE1 is active, while the AC from
   CE1 to PE3 is standby, as determined by the redundancy protocol
   running on the ACs. Thus, in normal operation, PE1 and PE3 will
   advertise "Active" and "Standby" 'Preferential Forwarding' status bit
   respectively to PE2, reflecting the forwarding state of the two ACs
   to CE1 as determined by the AC dual-homing protocol. PE2 advertises
   'Preferential Forwarding' status bit of "Active" on both PW1 and PW2
   since the AC to CE2 is single homed. As both the local and remote
   UP/DOWN status and preferential forwarding status for PW1 are UP and
   Active, traffic is forwarded over PW1 in both directions.

   On failure of the AC between CE1 and PE1, the forwarding state of the
   AC on PE3 transitions to Active. PE3 then announces the newly changed
   'Preferential Forwarding' status bit of "Active" to PE2. PE1 will
   advertise a PW status notification message indicating that the AC
   between CE1 and PE1 is DOWN. PE2 matches the local and remote
   preferential forwarding status of "Active" and status of "Pseudowire
   forwarding" and select PW2 as the new active pseudowire to send
   traffic to.

   On failure of PE1 node, PE3 will detect it and will transition the
   forwarding state of its AC to Active. The method by which PE3 detects
   that PE1 is down is outside the scope of this document. PE3 then
   announces the newly changed 'Preferential Forwarding' status bit of
   "Active" to PE2. PE3 and PE2 match the local and remote preferential
   forwarding status of "Active" and UP/DOWN status "Pseudowire
   forwarding" and select PW2 as the new active pseudowire to send
   traffic to. Note that PE2 may have detected that the PW to PE1 went
   down via T-LDP Hello timeout or via other means. However, it will not
   be able to forward user traffic until it receives the updated status
   bit from PE3.

   Note in this example, the receipt of the AC status on the CE1-PE1
   link is normally sufficient for PE2 to switch to PW2. However, the
   operator may want to trigger the switchover of the PW for
   administrative reasons, e.g., maintenance, and thus the use of the
   'Preferential Forwarding' status bit is required to notify PE2 to
   trigger the switchover.




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   Note that the primary/secondary procedures do not apply in this case
   as the PW PW Preferential Forwarding status is driven by the AC
   forwarding state as determined by the AC dual-homing protocol used.

15.2. Multiple Multi-homed CEs with single SS-PW redundancy

             |<-------------- Emulated Service ---------------->|
             |                                                  |
             |          |<------- Pseudo Wire ------>|          |
             |          |                            |          |
             |          |    |<-- PSN Tunnels-->|    |          |
             |          V    V    (not shown)   V    V          |
             V    AC    +----+                  +----+     AC   V
       +-----+    |     |....|.......PW1........|....|     |    +-----+
       |     |----------| PE1|......   .........| PE3|----------|     |
       | CE1 |          +----+      \ /  PW3    +----+          | CE2 |
       |     |          +----+       X          +----+          |     |
       |     |          |    |....../ \..PW4....|    |          |     |
       |     |----------| PE2|                  | PE4|--------- |     |
       +-----+    |     |....|.....PW2..........|....|     |    +-----+
                  AC    +----+                  +----+    AC


     Figure 15-2 Multiple Multi-homed CEs with single SS-PW redundancy

   The application in Figure 15-2 makes use of the Independent mode of
   operation.

   CE1 is dual-homed to PE1 and PE2. CE2 is dual-homed PE3 and PE4. The
   method for dual-homing and the used protocols 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.

   Assume that the AC from CE1 to PE1 is Active, from CE1 to PE2 is
   Standby; furthermore, assume that the AC from CE2 to PE3 is Standby
   and from CE2 to PE4 is Active. The method of deriving Active/Standby
   status of the AC is outside the scope of this document.

   PE1 advertises the preferential status "Active" and UP/DOWN status
   "Pseudowire forwarding" for pseudowires PW1 and PW4 connected to PE3
   and PE4. This status reflects the forwarding state of the AC
   attached to PE1. PE2 advertises preferential status "Standby" and
   UP/DOWN status "Pseudowire forwarding" for pseudowires PW2 and PW3
   to PE3 and PE4. PE3 advertises preferential status "Standby" and
   UP/DOWN status "Pseudowire forwarding" for pseudowires PW1 and PW3
   to PE1 and PE2. PE4 advertise the preferential status "Active" and


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   UP/DOWN status "Pseudowire forwarding" for pseudowires PW2 and PW4
   to PE2 and PE1 respectively. Thus by matching the local and remote
   preferential forwarding status of "Active" and UP/DOWN status of
   "Pseudowire forwarding" of pseudowires, the PE nodes determine which
   PW should be in the Active state. In this case it is PW4 that will
   be selected.


   On failure of the AC between CE1 and PE1, the forwarding state of
   the AC on PE2 is changed to Active. PE2 then announces the newly
   changed 'Preferential Forwarding' status bit of "active" to PE3 and
   PE4. PE1 will advertise a PW status notification message indicating
   that the AC between CE1 and PE1 is down. PE2 and PE4 match the local
   and remote preferential forwarding status of "Active" and UP/DOWN
   status "Pseudowire forwarding" and select PW2 as the new active
   pseudowire to send traffic to.


   On failure of PE1 node, PE2 will detect it and will transition the
   forwarding state of its AC to Active. The method by which PE2
   detects that PE1 is down is outside the scope of this document. PE2
   then announces the newly changed 'Preferential Forwarding' status
   bit of "Active" to PE3 and PE4. PE2 and PE4 match the local and
   remote preferential forwarding status of "Active" and UP/DOWN status
   "Pseudowire forwarding" and select PW2 as the new active pseudowire
   to send traffic to. Note that PE3 and PE4 may have detected that the
   PW to PE1 went down via T-LDP Hello timeout or via other means.
   However, they will not be able to forward user traffic until they
   received the updated status bit from PE2.


   Because each dual-homing algorithm running on the two node sets,
   i.e., {CE1, PE1, PE2} and {CE2, PE3, PE4}, selects the active AC
   independently, there is a need to signal the active status of the AC
   such that the PE nodes can select a common active PW for end-to-end
   forwarding between CE1 and CE2 as per the procedures in the
   independent mode.

   Note that any primary/secondary procedures, as defined in sections
   5.1.  and 5.2. , do not apply in this use case as the Active/Standby
   status is driven by the AC forwarding state as determined by the AC
   dual-homing protocol used.

15.3. Multi-homed CE with MS-PW redundancy

   The following figure illustrates an application of multi-segment
   pseudowire redundancy.


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           Native   |<-----------Pseudo Wire----------->|  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|=========|T-PE4|   |   +----+
                                    +-----+         +-----+   AC



           Figure 15-3 Multi-homed CE with MS-PW redundancy

   The application in Figure 15-3 makes use of the Independent mode of
   operation.

   CE2 is dual-homed to T-PE2 and T-PE4. PW1 and PW2 are used to extend
   the resilient connectivity all the way to T-PE1. PW1 has two segments
   and is active pseudowire while PW2 has two segments and is a standby
   pseudowire. This application requires support for MS-PW with segments
   of the same type as described in [4].

   The operation in this case is the same as in the case of SS-PW as
   described in Section 15.1. . The only difference is that the S-PE
   nodes need to relay the PW status notification containing both the
   UP/DOWN and forwarding status to the T-PE nodes.

15.4. Multi-homed CE with MS-PW redundancy and S-PE protection

   The following figure illustrates an application of multi-segment
   pseudowire redundancy with 1:1 PW protection.












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              Native   |<-----------Pseudo Wire------------->|  Native
              Service  |                                     |  Service
               (AC)    |     |<-PSN1-->|     |<-PSN2-->|     |   (AC)
                 |     V     V         V     V         V     V    |
                 |                     +-----+                    |
                 |       |=============|     |=============|      |
                 |       |.....PW3-Seg1......|.PW3-Seg2....|      |
                 |       |.|===========|S-PE3|===========|.|      |
                 |       |.|           +-----+           |.|      |
                 |     +-----+         +-----+         +-----+    |
          +----+ |     |T-PE1|=========|S-PE1|=========|T-PE2|    |  +----+
          |    |-------|......PW1-Seg1.......|PW1-Seg2.......|-------|    |
          |    |       |     |=========|     |=========|     |       |    |
          | CE1|       +-----+         +-----+         +-----+       |    |
          |    |       |.| |.|         +-----+         +-----+       | CE2|
          |    |       |.| |.|=========|     |=========|     |       |    |
          |    |       |.| |...PW2-Seg1......|.PW2-Seg2......|-------|    |
          +----+       |.| |===========|S-PE2|=========|T-PE4|    |  +----+
                       |.|             +-----+         +-----+    AC
                       |.|             +-----+           |.|
                       |.|=============|     |===========|.|
                       |.......PW4-Seg1......|.PW4-Seg2....|
                       |===============|S-PE4|=============|
                                       +-----+

     Figure 15-4  Multi-homed CE with MS-PW redundancy and protection

   The application in Figure Figure 15-4 makes use of the
   Independent mode of operation.

   CE2 is dual-homed to T-PE2 and T-PE4. The PW pairs {PW1,PW3} and
   {PW2, PW4} are used to extend the resilient connectivity all the
   way to T-PE1, like in the case in Section 15.3. , with the
   addition that this setup provides for S-PE node protection.

   CE1 is connected to T-PE1 while CE2 is dual-homed to T-PE2 and
   T-PE4. There are four segmented PWs. PW1 and PW2 are primary PWs
   and are used to support CE2 multi-homing. PW3 and PW4 are
   secondary PWs and are used to support 1:1 PW protection. PW1,
   PW2, PW3 and PW4 have two segments and they are switched at S-
   PE1, S-PE2, S-PE3 and S-PE4 respectively.

   It is possible that S-PE1 coincides with S-PE4 and/or SP-2
   coincides with S-PE3, in particular where the two PSN domains
   are interconnected via two nodes. However Figure 15-4 shows four
   separate S-PEs for clarity.



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   The behavior of this setup is exactly the same as in the setup
   in Section 15.3.  except that T-PE1 will always see a pair of
   PWs eligible for the active state, for example the pair
   {PW1,PW3} when the AC between CE2 and T-PE2 is in active state.
   Thus, it is important that both T-PE1 and T-PE2 implement a
   common mechanism to choose one the two PWs for forwarding as
   explained in Section 5.1. Similarly, T-PE1 and T-PE4 must use
   the same mechanism to select among the pair {PW2, PW4} when the
   AC between CE2 and T-PE4 is in active state.

15.5. Single Homed CE with MS-PW redundancy

   The following is an application of the independent mode of operation
   along with the optional request switchover procedures in order to
   provide N:1 PW protection. A revertive behavior to a primary PW is
   shown as an example of configuring and using the primary/secondary
   procedures described in sections 5.1.  and 5.2. .

           Native   |<------------Pseudo Wire------------>|  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 15-5 Single homed CE with multi-segment pseudowire redundancy

   CE1 is connected to PE1 in provider Edge 1 and CE2 to PE2 in provider
   edge 2 respectively. There are three segmented PWs. A primary PW,
   PW1, is switched at S-PE1. A primary PW, PW1 has the lowest
   precedence value of zero. A secondary PW, PW2, which is switched at
   S-PE2 and has a precedence of 1. Finally, another secondary PW, PW3,
   is switched at S-PE3 and has a precedence of 2. The precedence is


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   locally configured at the endpoints of the PW, i.e., T-PE1 and T-PE2.
   Lower the precedence value, higher the priority.

   T-PE1 and T-PE2 will select the PW they intend to activate based on
   their local and remote UP/DOWN state as well as the local precedence
   configuration. In this case, they will both advertise Preferential
   Forwarding' status bit of "Active" on PW1 and of "Standby" on PW2 and
   PW3 using priority derived from local precedence configuration.
   Assuming all PWs are UP, T-PE1 and T-PE2 will use PW1 to forward user
   packets.

   If PW1 fails, then the T-PE detecting the failure will send a status
   notification to the remote T-PE with a "Local PSN-facing PW (ingress)
   Receive Fault" bit set, or a "Local PSN-facing PW (egress) Transmit
   Fault" bit set, or a "Pseudowire Not Forwarding" bit set. In
   addition, it will set the 'Preferential Forwarding' status bit on PW1
   to "Standby". It will also advertise the 'Preferential Forwarding'
   status bit on PW2 as "Active" as it has the next lowest precedence
   value. T-PE2 will also perform the same steps as soon as it is
   informed of the failure of PW1. Both T-PE nodes will perform a match
   on the 'preferential forwarding' status of "Active" and UP/DOWN
   status of "Pseudowire forwarding" and will use PW2 to forward user
   packets.

   However this does not guarantee that the T-PEs will choose the same
   PW from the redundant set to forward on, for a given emulated
   service, at all times. This may be due to a mismatch of the
   configuration of the PW precedence in each T-PE. This may also be due
   to a failure which caused the endpoints to not be able to match the
   Active 'Preferential Forwarding' status bit and UP/DOWN status bits.
   In this case, T-PE1 and/or T-PE2 can invoke the optional request
   switchover/acknowledgement procedures to synchronize the choice of PW
   to forward on in both directions.

   The trigger for sending a request to switchover can also be the
   execution of an administrative maintenance operation by the network
   operator in order to move the traffic away from the T-PE/S-PE nodes
   /links to be serviced.

   In case the request switchover is sent by both endpoints
   simultaneously, both T-PEs send status notification with the newly
   selected PW with 'request switchover' bit set, waiting for response
   from the other endpoint. In such situation, the T-PE with greater
   system address request is given precedence. This helps in
   synchronizing PWs in event of mismatch of precedence configuration as
   well.



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         On recovery of primary PW1, PW1 is selected to forward traffic
   and the secondary PW, PW2, is set to standby.

15.6. PW redundancy between H-VPLS MTU-s and PE-rs

   Following figure illustrates the application of use of PW redundancy
   in H-VPLS for the purpose of dual-homing an MTU-s node to PE nodes
   using PW spokes. This application makes use of the Master/Slave mode
   of operation.


                         |<-PSN1-->|     |<-PSN2-->|
                         V         V     V         V
                   +-----+         +--------+
                   |MTU-s|=========|PE1-rs  |========
                   |..Active PW group....   | H-VPLS-core
                   |     |=========|        |=========
                   +-----+         +--------+
                      |.|
                      |.|           +--------+
                      |.|===========|        |==========
                      |...Standby PW group   |.H-VPLS-core
                       =============|  PE2-rs|==========
                                    +--------+

             Figure 15-6 Multi-homed MTU-s in H-VPLS core

   MTU-s is dual homed to PE1-rs and PE2-rs. The primary spoke PWs from
   MTU-s are connected to PE1-rs while the secondary PWs are connected
   to PE2. PE1-rs and PE2-rs are connected to H-VPLS core on the other
   side of network. MTU-s communicates to PE1-rs and PE2-rs the
   forwarding status of its member PWs for a set of VSIs having common
   status Active/Standby. It may be signaled using PW grouping with
   common group-id in PWid FEC Element or Grouping TLV in Generalized
   PWid FEC Element as defined in [2] to scale better.  MTU-s derives
   the status of the PWs based on local policy configuration. In this
   example, the primary/secondary procedures, as defined in Section 5.2.
   , are used but this can be based on any other policy.

   Whenever MTU-s performs a switchover, it sends a wildcard
   Notification Message to PE2-rs for the previously standby PW group
   containing PW Status TLV with PW Preferential Forwarding bit cleared.
   On receiving the notification PE-2rs unblocks all member PWs
   identified by the PW group and state of PW group changes from Standby
   to Active. All procedures described in Section 6.2. are applicable.




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   The use of the 'Preferential Forwarding' status bit in Master/Slave
   mode is similar to Topology Change Notification in RSTP controlled
   IEEE Ethernet Bridges but is restricted over a single hop. When these
   procedures are implemented, PE-rs devices are aware of switchovers at
   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 currently defined in
   [6] to prevent two copies of MAC withdraws to be sent, one by MTU-s
   and another one by PE-rs nodes. Mechanisms to disable MAC Withdraw
   trigger in certain devices is out of the scope of this document

Author's Addresses

   Praveen Muley
   Alcatel-lucent
   701 E. Middlefiled Road
   Mountain View, CA, USA
   Email: Praveen.muley@alcatel-lucent.com

   Mustapha Aissaoui
   Alcatel-lucent
   600 March Rd
   Kanata, ON, Canada K2K 2E6
   Email: mustapha.aissaoui@alcatel-lucent.com

























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