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Pseudowire Preferential Forwarding Status Bit
draft-ietf-pwe3-redundancy-bit-09

The information below is for an old version of the document that is already published as an RFC.
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This is an older version of an Internet-Draft that was ultimately published as RFC 6870.
Authors Praveen Muley , Mustapha Aissaoui
Last updated 2018-12-20 (Latest revision 2013-01-04)
Replaces draft-muley-dutta-pwe3-redundancy-bit
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draft-ietf-pwe3-redundancy-bit-09
Network Working Group                                Praveen Muley, Ed. 
Internet Draft                                   Mustapha Aissaoui, Ed.  
Updates: RFC 4447                                        Alcatel-Lucent 
Intended Status: Standards Track                                        
Expires: July 4, 2013                                   January 4, 2013 
                                      
                                      

               Pseudowire Preferential Forwarding Status Bit  
                   draft-ietf-pwe3-redundancy-bit-09.txt 

Abstract 

   This document describes a mechanism for signaling the active and 
   standby status 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 
   defined. This bit indicates a preferential forwarding status with a 
   value 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. 

   Finally, this document updates RFC 4447 by adding details to the 
   handling of the PW Status Code bits in the PW Status TLV. 

 

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." 

 
 
 
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   This Internet-Draft will expire on July 4, 2013. 

 

Copyright Notice 

   Copyright (c) 2013 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...........................................4 
   3. Terminology....................................................6 
   4. PE Architecture................................................8 
   5. Modes of Operation.............................................9 
      5.1. Independent Mode:.........................................9 
      5.2. Master/Slave Mode:.......................................12 
   6. PW State Transition Signaling Procedures......................14 
      6.1. PW Standby Notification Procedures in Independent mode...14 
      6.2. PW Standby notification procedures in Master/Slave mode..15 
         6.2.1. PW State Machine....................................15 
      6.3. Coordination of PW Switchover............................17 
         6.3.1. Procedures at the requesting endpoint...............18 
         6.3.2. Procedures at the receiving endpoint................19 
   7. Status Mapping................................................20 
 
 
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      7.1. AC Defect State Entry/Exit...............................20 
      7.2. PW Defect State Entry/Exit...............................20 
   8. Applicability and Backward Compatibility......................21 
   9. Security Considerations.......................................21 
   10. MIB Considerations...........................................22 
   11. IANA Considerations..........................................22 
      11.1. Status Code for PW Preferential Forwarding Status.......22 
      11.2. Status Code for PW Request Switchover Status............22 
   12. Contributors.................................................22 
   13. Acknowledgments..............................................23 
   14. References...................................................24 
      14.1. Normative References....................................24 
      14.2. Informative References..................................24 
   15. Appendix A - Applications of PW Redundancy Procedures........25 
      15.1. One Multi-homed CE with single SS-PW redundancy.........25 
      15.2. Multiple Multi-homed CEs with single SS-PW redundancy...27 
      15.3. Multi-homed CE with MS-PW redundancy....................28 
      15.4. Multi-homed CE with MS-PW redundancy and S-PE protection29 
      15.5. Single Homed CE with MS-PW redundancy...................31 
      15.6. PW redundancy between H-VPLS MTU-s and PE-rs............33 
   Authors' Addresses...............................................34 
    
1. Introduction 

   This document provides the extensions to the pseudowire (PW) control 
   plane to support the protection schemes of the PW redundancy 
   applications described in RFC6718 (PW Redundancy [8]). 

   It specifies a new PW status bit as well as the procedures provider 
   edge (PE) nodes follow to notify one another of the preferential 
   forwarding state of each PW in the redundant set i.e. active or 
   standby. This status bit is different from the PW status bits 
   already defined in RFC 4447, the pseudowire setup and maintenance 
   protocol [2]. In addition, this document specifies a second status 
   bit to allow peer PE nodes to coordinate a switchover operation of 
   the PW from active to standby, or vice versa. 

   As a result of the introduction of these new status bits, this 
   document updates RFC 4447 by clarifying the rules for processing 
   status bits not originally defined in RFC 4447. It also updates RFC 
   4447 by defining that status bit can indicate a status other than a 
   fault or can indicate an instruction to the peer PE. See more 
   details in Section 8.  

    

 
 
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   Section 15. 15 shows in detail how the mechanisms described in this 
   document are used to achieve the desired protection schemes of the 
   applications described in [8]. 

2. Motivation and Scope    

   The PW setup and maintenance protocol defines the following status 
   codes in the PW status TLV to indicate the state for an AC and a PW 
   [7]: 

   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 applications defined in [8] allow the provisioning of a primary 
   PW and one or many secondary backup PWs in the same Virtual Private 
   Wire Service (VPWS) or Virtual Private LAN Service (VPLS). The 
   objective of PW redundancy is to maintain end-to-end connectivity 
   for the emulated service by activating the correct PW whenever an 
   AC, a PE, or a PW fails. The correct PW means the one which provides 
   the end-to-end connectivity from CE to CE such that packets continue 
   to flow. 

   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, and the procedures the PE nodes 
   follow 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 protection capabilities defined in [8]: 

      a. A 1:1 protection in which a specific subset of a path for an 
         emulated service, consisting of a standby PW and/or AC, 
         protects another specific subset of a path for the emulated 
         service, consisting of an active PW and/or AC.  . An active PW 
         can forward data traffic and control plane traffic, such as 
         Operations, Administration, and Maintenance (OAM) packets. A 
         standby PW does not carry data traffic. 

      b. An N:1 protection scheme in which N specific subsets of a path 
         for an emulated service, consisting each of a standby PW 
         and/or AC, protects a specific subset of a path for the 
         emulated service, consisting of an active PW and/or AC.  . 

      c. A 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. 
         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. A locally configured precedence to govern the selection of a 
         PW when more than one PW qualifies for the active state, as 
         defined in sections 5.1. 5.1 and 5.2. 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. Implementations can designate by configuration one PW in the 
         1:1 or N:1 protection 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. 5.1 and 5.2. 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 effectively achieving revertive 
         protection switching. 

 
 
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   1+1 protection in which one specific subset of a path for an 
   emulated service, consisting of a standby PW and/or AC, protects 
   another specific subset of a path for the emulated service is not 
   supported.  

   The above protection schemes are provided using the following 
   operational modes: 

          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.  

   Note that this document specifies the mechanisms to support PW 
   redundancy where a set of redundant PWs terminate on either a PE, in 
   the case of a single-segment pseudowire (SS-PW), or on a terminating 
   provider edge (T-PE)in the case of a multi-segment pseudowire (MS-
   PW). PW redundancy scenarios where the redundant set of PW segments 
   terminates on an S-PE are for further study. 

3. Terminology 

   Pseudowire (PW): A mechanism that carries the essential elements              
            of an emulated service from one PE to one or                        
            more other PEs over a PSN [9]. 

   Single-Segment Pseudowire (SS-PW): A PW set up directly between      
            two T-PE devices.  The PW label is unchanged between the      
            originating and terminating T-PEs [6]. 

   Multi-Segment Pseudowire (MS-PW): A static or dynamically      
            configured set of two or more contiguous PW segments that 
            behave and function as a single point-to-point PW.  Each 
            end of an MS-PW, by definition, terminates on a T-PE [6].  

   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 [7]. Such a PW is available for 
            forwarding traffic [8]. 
 
 
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   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[7], such a PW is not available for forwarding 
            traffic [8].  

   Active PW:  An up PW used for forwarding user, OAM and control plane 
            traffic [8]. 

   Standby PW: An up PW that is not used for forwarding user traffic, 
           but may forward OAM and specific control plane traffic [8]. 

   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 and is only required when 
           revertive protection switching is used [8].  

   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 [8]. 

   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 [8]. 

   Provider Edge (PE): A device that provides PWE3 to a CE [9]. 

   PW Terminating Provider Edge (T-PE):  A PE where the customer-     
           facing attachment circuits (ACs) are bound to a PW 
           forwarder.  A terminating PE is present in the first and 
           last segments of an MS-PW.  This incorporates the 
           functionality of a PE as defined in RFC 3985 [6]. 

   PW Switching Provider Edge (S-PE): A PE capable of switching the      
           control and data planes of the preceding and succeeding PW 
           segments in an MS-PW.  The S-PE terminates the PSN tunnels 
 
 
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           of the preceding and succeeding segments of the MS-PW.  It 
           therefore includes a PW switching point for an MS-PW.  A PW 
           switching point is never the S-PE and the T-PE for the same 
           MS-PW.  A PW switching point runs necessary protocols to set 
           up and manage PW segments with other PW switching points and 
           terminating PEs.  An S-PE can exist anywhere a PW must be 
           processed or policy applied.  It is therefore not     
           limited to the edge of a provider network [6]. 

   MTU-s: A hierarchical virtual private LAN service Multi-Tenant      
           Unit switch, as defined in RFC 4762 [3]. 

   PE-rs: A routing and bridging capable PE as defined in RFC 4762 [3]. 

   FEC: Forwarding Equivalence Class. 

   OAM:    Operations, Administration, and Maintenance. 

   VCCV:   Virtual Connection Connectivity Verification. 

   This document uses the term 'PE' to be synonymous with both PEs as 
           per RFC 3985 [9] and T-PEs as per RFC 5659 [6]. 

   This document uses the term 'PW' to be synonymous with both PWs as 
           per RFC 3985 [9] and SS-PWs, MS-PWs, and PW segments as per 
           RFC 5659 [6]. 

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 RFC 3985 [9]. 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 Architecture 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 PWs are eligible to 
   become active and which are not. They advertise the corresponding 
   Active or 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. 
   The default mechanism MUST be supported by all implementations and 
   operates as follows: 

 
 
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   1. For a PW ID Forwarding Equivalence Class (PWid FEC) PW [2], the 
      PW with the lowest pw-id value is selected. 

   2. For a Generalized PWid FEC PW [2], each PW in a redundant set is 
      uniquely identified at each PE using the following triplet: 
      AGI::SAII::TAII. The unsigned integer form of the concatenated 
      word can be used in the comparison. However, the SAII and TAII 
      values as seen on a PE node are the mirror values of what the 
      peer PE node sees. So that both PE nodes compare the same value, 
      the PE with the lowest system IP address MUST use the unsigned 
      integer form of AGI::SAII::TAII while the PE with the highest 
      system IP address MUST use the unsigned integer form of 
      AGI::TAII::SAII. This way, both PE nodes will compare the same 
      values. The PW which corresponds to the minimum of the compared 
      values across all PWs in the redundant set is selected. 

      In the case where the system IP address is not known, it is 
      RECOMMENDED to implement the active PW selection mechanism 
      described next. 

      In the case of segmented PW, the operator needs to make sure that 
      the pw-id or AGI::SAII::TAII of the redundant PWs within the 
      first and last segment are ordered consistently such that the 
      same end-to-end MS-PW gets selected. Otherwise, it is RECOMMENDED 
      to implement the active PW selection mechanism described next. 

   The PW endpoints MAY also implement the following active PW 
   selection mechanism.  

   1. If the PW endpoint is configured with the precedence parameter on 
      each PW in the redundant set, it selects the PW with the lowest 
      configured precedence value.  

   2. If the PW endpoint is configured with one PW as primary and one 
      or more PWs as secondary, it selects the primary PW in preference 
      to all secondary PWs. If a primary PW is not available, it 
      selects the secondary PW with the lowest precedence value. If the 
      primary PW becomes available, a PW endpoint reverts to it 
      immediately or after the expiration of a configurable delay. 

   3. This active PW selection mechanism assumes the precedence 
      parameter values are configured consistently at both PW endpoints 
      and that unique values are assigned to the PWs in the same 
      redundant set to achieve tie-breaking using this mechanism. 

   There are scenarios with dual-homing of a CE to PE nodes where each 
   PE node needs to advertise Active preferential forwarding status on 
 
 
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   more than one PW in the redundant set. However, a PE MUST always 
   select a single PW for forwarding using the above active PW 
   selection algorithm. An example of such a case is described in 15.2. 
   . 

   There are scenarios where each PE needs to advertize Active 
   preferential forwarding status on a single PW in the redundant set. 
   In order to ensure that both PE nodes make the same selection, they 
   MUST use the above active PW selection algorithm to determine the PW 
   eligible for active state. An example of such a case is described in 
   15.5. . 

   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 notification SHOULD be generated. If a 
   management notification for failure to find an active PW was 
   generated and an active PW is subsequently found, a management 
   notification SHOULD be generated, so clearing the previous failure 
   indication. Additionally, a PE MAY use the request switchover 
   procedures described in Section 6.3. 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. 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 
 
 
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   status bits exchanged indicate the PW preferential forwarding status 
   is Standby at one or both endpoints. In this state the endpoints 
   MUST NOT forward data traffic over the PW but MAY allow PW OAM 
   packets, e.g., Virtual Connection Connectivity Verification (VCCV) 
   packets [12], 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 RFC 4762 [3] and in [11]. 

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.  

   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   
   behavior described in this section assumes correct configuration of   
   the Master and Slave endpoints. This document does not define a    
   mechanism to detect errors in the configuration, and 
   misconfiguration might lead to protection switchover failing to work 
   correctly. Furthermore, this document does not specify the 
   procedures for a backup Master node. In deployments where PE node 
   protection is required, it recommended to use the Independent Mode 
   of operation as in the application described in 15.2.  

   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 redundant 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.  
 
 
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   If more than one PW qualifies for the Active state, the Master PW 
   endpoint node selects one. There is no requirement to specify a 
   default active PW selection mechanism in this case but for 
   consistency across implementations, the Master PW endpoint SHOULD 
   implement the default active PW selection mechanism described in 
   Section 5.1.  

   If the Master PW endpoint implements the active PW selection 
   mechanism based on primary/secondary and precedence parameters, it 
   MUST follow the following behavior:  

   1. If the PW endpoint is configured with the precedence parameter on       
      each PW in the redundant set, it MUST select the PW with the       
      lowest configured precedence value. 

   2. If the PW endpoint is configured with one PW as primary and one   
      or more PWs as secondary, it MUST select 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. 

   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.  

   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 
 
 
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   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 RFC 4762 [3] and [11]. 

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. 

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 [8]). 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 
   as explained in Section 5.1. 5.1 . 

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

 
 
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   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 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, 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 
   PWid FEC TLV with only the Group ID, or else it contains the 
   Generalized PWid 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 
 
 
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   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.  

    
                      
    
      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: None 
    
    
          Table 1  PW State Transition Table in Master/Slave Mode 

 
 
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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 
   redundant MS-PW is identified in [8]. 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 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 redundant 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. 

   A PE MAY support the 'request switchover' bit. A PE which receives 
   the 'request switchover' bit and which does not support it will 
   ignore it.  

   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.  

 
 
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   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]. 

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. This timer SHOULD be 
      configurable with a default value of 3 seconds. 

   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 

 
 
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               '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: 

            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 
 
 
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               '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 
               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 [5] with the following modifications.  

7.1. AC Defect State Entry/Exit 

   A PE enters the AC receive (or transmit) defect state for a PW 
   service when one or more of the conditions specified for this PW 
   service in [5] 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 redundant set which are associated with 
   this AC. 

   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 redundant set which are 
   associated with this AC. 

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 [5] are met for each of the PWs in the 
   redundant set. 

 
 
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   When a PE enters the PW receive (or transmit) defect state for a PW   
   service associated with an AC, it MUST send a reverse (or forward)   
   defect indication over one or more of the PWs in the redundant set   
   associated with the same AC if the PW failure was detected by this   
   PE without receiving a forward defect indication from the remote   
   PE [5]. 

   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 redundant associated with the same AC set if applicable. 

8. Applicability and Backward Compatibility 

   The mechanisms defined in this document are to be used in 
   applications where standby state signaling of a PW or PW group is 
   required. Both PWid FEC and Generalized PWid FEC are supported. All 
   PWs which are part of a redundant set MUST use the same FEC type. 
   When the set uses PWid FEC element, each PW is uniquely identified 
   by its PW ID. When the redundant set uses Generalized PWid FEC 
   element, 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, MUST ignore 
   these status bits if they are set  by a peer PE. This document in 
   fact updates RFC 4447 by prescribing the same behavior for any 
   status bit not originally defined in RFC 4447. 

   Finally this document updates RFC 4447 by defining that status bit 
   can indicate a status other than a fault or can indicate an 
   instruction to the peer PE. As a result, a PE implementation 
   compliant to RFC 4447 MUST process each status bit it supports when 
   set according to the rules specific to that status bit. 

9. Security Considerations  

   LDP extensions/options that protect pseudowires must be   
   implemented because the status bits defined in this document have 
   the same security considerations as the pseudowire setup and 
 
 
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   maintenance protocol defined in RFC4447 [2]. It should be noted that 
   the security of a PW redundant set is only as good as the weakest 
   security on any of its members. 

10. MIB Considerations 

   New MIB objects for the support of PW redundancy will be defined in 
   a separate document. 

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  

              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. Contributors 

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

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

 
 
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   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 
   Juniper Networks 
   Email: tnadeau@lucidvision.com  

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

   Hamid Ould-Brahim 
   Email: ouldh@yahoo.com    

   Olen Stokes  
   Extreme Networks  
   Email: ostokes@extremenetworks.com 

   Daniel Cohn  
   Orckit  
   daniel.cohn.ietf@gmail.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, 
   and Nicolai Leymann. 

 
 
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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. 

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

   [5]   Aissaoui, M., et al., "Pseudowire (PW) OAM Message Mapping", 
         RFC 6310, July 2011. 

    

14.2. Informative References 

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

   [7]   Martini, L., "IANA Allocations for Pseudowire Edge to Edge              
         Emulation (PWE3)", BCP 116, RFC 4446, April 2006. 

   [8]   Muley, P., et al., "Pseudowire (PW) Redundancy",  RFC 6718, 
         August 2012.  

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

   [10]  Nadeau, T., Zelig, D., Nicklass, O., "Definitions of Textual 
         Conventions for Pseudowire (PW) Management", RFC 5542, May 
         2009 

   [11]  Dutta, P., Lasserre, M., Stokes, O., "LDP Extensions for 
         Optimized MAC Address Withdrawal in H-VPLS", draft-ietf-l2vpn-
         vpls-ldp-mac-opt-05.txt, October 2011 

   [12]  Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit 
         Connectivity Verification (VCCV): A Control Channel for 
         Pseudowires", RFC 5085, December 2007. 
 
 
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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 some of the 
   applications described in the PW Redundancy [8]. 

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

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

         |<-------------- Emulated Service ---------------->|  
         |                                                  |  
         |          |<------- Pseudowire  ------>|          |  
         |          |                            |          |  
         |          |    |<-- 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 

   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 [8]). 

   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 
 
 
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   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.  

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

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

             |<-------------- Emulated Service ---------------->|  
             |                                                  |  
             |          |<------- Pseudowire  ------>|          |  
             |          |                            |          |  
             |          |    |<-- 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 
   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 
 
 
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   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. 5.1  and 5.2. 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   |<-----------Pseudowire ------------->| 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. It extends the application described in Section 15.1. 
   15.1 of this document and in [8] by adding a pair of S-PE nodes to 
   switch the segments of PW1 and PW2. 

   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. 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   |<-----------Pseudowire ------------->|  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 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. 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-PE nodes for clarity. 

 
 
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   The behavior of this setup is exactly the same as in the setup 
   in Section 15.3. 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 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. 5.1 and 5.2. 5.2 . 
    
          Native   |<------------Pseudowire ------------>|  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. 
 
 
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   The precedence is 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 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. 

 
                                                   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 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 
 
 
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   example, the primary/secondary procedures as defined in Section 5.2. 
   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. 6.2 are 
   applicable. 

   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 [3] 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 

Authors' Addresses 

   Praveen Muley 
   Alcatel-lucent 
   701 E. Middlefield 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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