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Versions: (draft-hemige-serbest-l2vpn-vpls-pim-snooping) 00 01 02 03 04 05 06 07 draft-ietf-pals-vpls-pim-snooping

Layer 2 Virtual Private Networks                               O. Dornon
Internet-Draft                                               J. Kotalwar
Intended status: Informational                            Alcatel-Lucent
Expires: January 17, 2013                                       J. Zhang
                                                  Juniper Networks, Inc.
                                                               V. Hemige
                                                          Alcatel-Lucent
                                                           July 16, 2012


                         PIM Snooping over VPLS
                 draft-ietf-l2vpn-vpls-pim-snooping-02

Abstract

   In Virtual Private LAN Service (VPLS), as also in IEEE Bridged
   Networks, the switches simply flood multicast traffic on all ports in
   the LAN by default.  IGMP Snooping is commonly deployed to ensure
   multicast traffic is not forwarded on ports without IGMP receivers.
   The procedures and recommendations for IGMP Snooping are defined in
   [IGMP-SNOOP].  But when any protocol other than IGMP is used, the
   common practice is to simply flood multicast traffic to all ports.
   PIM-SM, PIM-SSM, PIM-BIDIR are widely deployed routing protocols.
   PIM Snooping procedures are important to restrict multicast traffic
   to only the routers interested in receiving such traffic.

   While most of the PIM Snooping procedures defined here also apply to
   IEEE Bridged Networks, VPLS demands certain special procedures due to
   the split-horizon rules that require the Provider Edge (PE) devices
   to co-operate.  This document describes the procedures and
   recommendations for PIM-Snooping in VPLS to facilitate replication to
   only those ports behind which there are interested PIM routers and/or
   IGMP hosts.  This document also describes procedures for PIM Proxy.
   PIM Proxy is required on PEs for VPLS Multicast to work correctly
   when Join suppression is enabled in the VPLS.  PIM Proxy also helps
   scale VPLS Multicast much better than just PIM Snooping.

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 [RFC2119].

Status of this Memo

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




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   Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

   Copyright (c) 2012 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
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   described in the Simplified BSD License.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . .  6
     1.2.  PIM Snooping and PIM Proxy Complexity  . . . . . . . . . .  6
     1.3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Multicast Traffic over VPLS  . . . . . . . . . . . . . . . . .  7
     2.1.  Constraining of IP Multicast in a VPLS . . . . . . . . . .  8
     2.2.  IPv6 Considerations  . . . . . . . . . . . . . . . . . . .  9
     2.3.  PIM-SM (*,*,RP) Considerations . . . . . . . . . . . . . .  9
     2.4.  PIM Packet Types to Snoop  . . . . . . . . . . . . . . . .  9
     2.5.  PIM Snooping vs PIM Proxy  . . . . . . . . . . . . . . . .  9
       2.5.1.  Differences between PIM Snooping and PIM Proxy . . . . 10
       2.5.2.  PIM Control Message Latency  . . . . . . . . . . . . . 11
       2.5.3.  When to Snoop and When to Proxy  . . . . . . . . . . . 11
   3.  PIM Snooping for VPLS  . . . . . . . . . . . . . . . . . . . . 12
     3.1.  General Rules for PIM Snooping in VPLS . . . . . . . . . . 13
       3.1.1.  Snooping PIM Packets . . . . . . . . . . . . . . . . . 13
       3.1.2.  Preserving Assert Trigger  . . . . . . . . . . . . . . 13
     3.2.  Discovering PIM Routers  . . . . . . . . . . . . . . . . . 14
     3.3.  PIM-SM and PIM-SSM . . . . . . . . . . . . . . . . . . . . 15
       3.3.1.  Building PIM-SM Snooping States  . . . . . . . . . . . 15
       3.3.2.  Explanation for per (S,G,N) states . . . . . . . . . . 17
       3.3.3.  Receiving (*,G) PIM-SM Join/Prune Messages . . . . . . 18
       3.3.4.  Receiving (S,G) PIM-SM Join/Prune Messages . . . . . . 20
       3.3.5.  Receiving (S,G,rpt) Join/Prune Messages  . . . . . . . 22
       3.3.6.  Sending Join/Prune Messages Upstream . . . . . . . . . 22
     3.4.  Bidirectional-PIM (PIM-BIDIR)  . . . . . . . . . . . . . . 23
     3.5.  Interaction with IGMP Snooping . . . . . . . . . . . . . . 24
     3.6.  PIM-DM . . . . . . . . . . . . . . . . . . . . . . . . . . 24
       3.6.1.  Building PIM-DM Snooping States  . . . . . . . . . . . 24
       3.6.2.  PIM-DM Downstream Per-Port PIM(S,G,N) State Machine  . 25
       3.6.3.  Triggering ASSERT election in PIM-DM . . . . . . . . . 25
     3.7.  PIM Proxy  . . . . . . . . . . . . . . . . . . . . . . . . 25
       3.7.1.  Downstream PIM Proxy behavior  . . . . . . . . . . . . 26
       3.7.2.  Upstream PIM Proxy behavior  . . . . . . . . . . . . . 26
       3.7.3.  Source IP Address in Proxy PIM Join/Prune Packets  . . 26
     3.8.  Directly Connected Multicast Source  . . . . . . . . . . . 27
     3.9.  Data Forwarding Rules  . . . . . . . . . . . . . . . . . . 27
       3.9.1.  PIM-SM Data Forwarding Rules . . . . . . . . . . . . . 28
       3.9.2.  PIM-BIDIR Data Forwarding Rules  . . . . . . . . . . . 29
       3.9.3.  PIM-DM Data Forwarding Rules . . . . . . . . . . . . . 30
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 31
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 31
   6.  Contributers . . . . . . . . . . . . . . . . . . . . . . . . . 31
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 32



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     8.2.  Informative References . . . . . . . . . . . . . . . . . . 32
   Appendix A.  PIM-BIDIR Thoughts  . . . . . . . . . . . . . . . . . 33
   Appendix B.  Example Network Scenario  . . . . . . . . . . . . . . 33
     B.1.  Pim Snooping Example . . . . . . . . . . . . . . . . . . . 34
     B.2.  PIM Proxy Example with (S,G) / (*,G) interaction . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39













































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1.  Introduction

   In Virtual Private LAN Service (VPLS), the Provider Edge (PE) devices
   provide a logical interconnect such that Customer Edge (CE) devices
   belonging to a specific VPLS instance appear to be connected by a
   single LAN.  Forwarding information base for particular VPLS instance
   is populated dynamically by source MAC address learning.  This is a
   straightforward solution to support unicast traffic, with reasonable
   flooding for unicast unknown traffic.  Since a VPLS provides LAN
   emulation for IEEE bridges as wells as for routers, the unicast and
   multicast traffic need to follow the same path for layer-2 protocols
   to work properly.  As such, multicast traffic is treated as broadcast
   traffic and is flooded to every site in the VPLS instance.  VPLS
   solutions (i.e., [VPLS-LDP] and [VPLS-BGP]) perform replication for
   multicast traffic at the ingress PE devices.  As stated in the VPLS
   Multicast Requirements draft [VPLS-MCAST-REQ], there are two issues
   with VPLS Multicast today: A. Multicast traffic is replicated to non-
   member sites.  B. Replication of PWs on shared physical path.

   This document solves Issue A of [VPLS-MCAST-REQ] by ensuring that IP
   multicast traffic is not forwarded to non-member sites.  Issue B is
   outside the scope of this document.  The different mechanisms to
   tunnel IP multicast traffic in a VPLS from the ingress PE to the
   egress PEs are discussed in[VPLS-MCAST-TREES].  The solution in this
   document when combined with the solutions proposed in the working
   group to solve Issue B will provide a complete VPLS Multicast
   solution set.

   Using IGMP/PIM Snooping in VPLS has the following advantages:

   o  It improves IP Multicast bandwidth usage in the VPLS core by
      ensuring traffic is replicated only to PEs with member sites.
      Note that this is not necessarily optimum, as there can still be
      bandwidth waste if traffic from a PE to other PE(s) is not
      forwarded along a minimum cost spanning tree.

   o  It prevents sending multicast traffic to non-member sites.

   Procedures for IGMP Snooping are specified in[IGMP-SNOOP].  This
   document describes the procedures for Protocol Independent Multicast
   (PIM) snooping over VPLS for efficient distribution of IP multicast
   traffic.  It also describes the rules when both IGMP and PIM are
   active in a VPLS instance.

   This document also describes procedures for PIM Proxy.  PIM Proxy is
   required on PEs for VPLS Multicast to work correctly when Join
   suppression is enabled in the VPLS.  PIM Proxy also helps scale VPLS
   Multicast much better than just PIM Snooping.



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1.1.  Assumptions

   Since this draft describes the procedures for PIM Snooping and PIM
   Proxy, the draft assumes that the reader has a good understanding of
   the PIM protocols.  The text in this draft is written in the same
   style as the PIM RFCs to help correlate the concepts and to make it
   easier to follow.  In order to avoid replicating the text relating to
   PIM protocol handling here, this draft assumes that the user will
   infer such detail from the PIM RFC referenced in this document.
   Deviations in protocol handling specific to PIM Snooping and PIM
   Proxy are specified in this draft.  There could be cross references
   into definitions of macros and procedures from the PIM RFCs.

1.2.  PIM Snooping and PIM Proxy Complexity

   The PIM Snooping and PIM Proxy solutions described here requires a
   switch to examine and operate on only PIM Hello and PIM Join/Prune
   packets.  The switch does not need to examine any other PIM packets.

   The switch does not need to have any routing tables like is required
   in PIM Multicast Routing.  It knows how to forward Join/Prunes by
   looking at the Upstream Neighbor field in the Join/Prune packets.

   The switch does not need to know about Rendezvous Points (RP) and
   does not have to maintain any RP Set. All that is transparent to a
   PIM Snooping switch.

   Most of the procedures in PIM Snooping and PIM Proxy in the handling
   of PIM Hellos and PIM Join/Prune packets are very similar to that of
   a PIM Router.

   The solutions described here provide complete separation of control
   and data planes.

   A PIM Proxy solution minimizes the control plane messages received at
   CE routers by proxying one message upstream on behalf of a large
   number of downstream CEs.  As such control plane messaging is very
   similar to that of a PIM Router.

1.3.  Definitions

   There are several definitions referenced in this document that are
   well described in the PIM RFCs [PIM-SM], PIM-BIDIR, PIM-DM].  The
   following definitions and abbreviations are used throughout this
   document:






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   o  A port is defined as either an attachment circuit (AC) or a
      Pseudo-Wire (PW).

   o  When we say a PIM message is 'received' on a port, it means that a
      PIM Snooping switch snooped the PIM message.

   Abbreviations used in the document:

   o  S: IP Address of the Multicast Source.

   o  G: IP Address of the Multicast Group.

   o  N: Upstream Neighbor field in a Join/Prune/Graft message.

   o  Rport(N): Port on which neighbor N is learnt

   Other definitions are explained in the sections where they are
   introduced.


2.  Multicast Traffic over VPLS

   In VPLS, if a PE receives a frame from an Attachment Circuit (AC)
   with no matching entry in the forwarding information base for that
   particular VPLS instance, it floods the frame to all other PEs (which
   are part of this VPLS instance) and to directly connected ACs (other
   than the one that the frame is received from).  The flooding of a
   frame occurs when:

   o  The destination MAC address has not been learned,

   o  The destination MAC address is a broadcast address,

   o  The destination MAC address is a multicast address.

   Malicious attacks (e.g., receiving unknown frames constantly) aside,
   the first situation is handled by VPLS solutions as long as
   destination MAC address can be learned.  After that point on, the
   frames will not be flooded.  A PE is REQUIRED to have safeguards,
   such as unknown unicast limiting and MAC table limiting, against
   malicious unknown unicast attacks.

   There is no way around flooding broadcast frames.  To prevent runaway
   broadcast traffic from adversely affecting the VPLS service and the
   SP network, a PE is REQUIRED to have tools to rate limit the
   broadcast traffic as well.

   Similar to broadcast frames, multicast frames are flooded as well, as



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   a PE cannot know where multicast members reside.  Rate limiting
   multicast traffic, while possible, should be done carefully since
   several network control protocols relies on multicast.  For one
   thing, layer-2 and layer-3 protocols utilize multicast for their
   operation.  For instance, Bridge Protocol Data Units (BPDUs) use an
   IEEE assigned all bridges multicast MAC address, and OSPF is
   multicast to all OSPF routers multicast MAC address.  If the rate-
   limiting of multicast traffic is not done properly, the customer
   network will experience instability and poor performance.  For the
   other, it is not straightforward to determine the right rate limiting
   parameters for multicast.

   A VPLS solution MUST NOT affect the operation of customer layer-2
   protocols (e.g., BPDUs).  Additionally, a VPLS solution MUST NOT
   affect the operation of layer-3 protocols.

   In the following section, we describe procedures to constrain the
   flooding of IP multicast traffic in a VPLS.

2.1.  Constraining of IP Multicast in a VPLS

   For a PE in a VPLS (a layer-2 device) to constrain IP multicast
   traffic, it needs to be able to learn which CEs are interested in
   receiving multicast traffic for what flows.

   The most obvious solution is to snoop IP multicast control traffic at
   the PEs.  Snooping as a solution to constrain multicast traffic makes
   sense under the following circumstances:

   o  The CE-CE protocol the PEs snoop is a popular and widely deployed
      protocol.

   o  It does not require any changes on the CEs and it should be
      completely transparent to the CEs.

   IGMP/MLD and PIM are the popular IP Multicast Routing protocols
   today.  Other routing protocols such as DVMRP or MOSPF are outside
   the scope of this document.

   This document describes the guidelines for PIM Snooping and PIM Proxy
   in VPLS.  The specifications in this document could be used for
   either PIM Snooping or PIM Proxy.  The PIM Proxy solution is
   described in section Section 3.7.  Differences that need to be
   observed while implementing one or the other and recommendations on
   which method to employ in different scenarios are noted in section
   Section 2.5.  We will largely refer to PIM "Snooping" in this
   document.  Unless specifically specified, the same procedures should
   apply to a Proxy solution as well.



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   In the following sub-sections, we provide some guidelines for the
   implementation of PIM snooping in VPLS.  Snooping techniques need to
   be employed on ACs at the downstream PEs.  Snooping techniques can
   also be employed on PWs at the upstream PEs.  This may work well for
   small to medium scale deployments.  However, if there are a large
   number of VPLS instances with a large number of PEs per instances,
   then the amount of snooping required at the upstream PEs can
   overwhelm the upstream PEs.

2.2.  IPv6 Considerations

   In VPLS, PEs forward Ethernet frames received from CEs and as such
   are agnostic of the layer-3 protocol used by the CEs.  However, as an
   IGMP and PIM snooping switch, the PE would have to look deeper into
   the IP and IGMP/PIM packets and build snooping state based on that.
   The PIM Protocol specifications handle both IPv4 and IPv6.  The
   specification for PIM Snooping in this draft can be applied to both
   IPv4 and IPv6 payloads.

2.3.  PIM-SM (*,*,RP) Considerations

   This draft does not address (*,*,RP) states in the VPLS network.
   Although [PIM-SM] specifies that routers MUST support (*,*,RP)
   states, there are very few implementations that actually support it
   in actual deployments.  Given the complexity of supporting (*,*,RP)
   states and knowing that there is little to no use to supporting it,
   this draft omits the specification relating to (*,*,RP) support.

2.4.  PIM Packet Types to Snoop

   A PIM Snooping switch need only snoop on PIM Hellos and PIM Join/
   Prune packets.  All other PIM packets can be transparently flooded
   unexamined.

2.5.  PIM Snooping vs PIM Proxy

   PIM Snooping switches simply snoop on PIM packets as they are being
   forwarded in the VPLS.  As such it truly provides transparent LAN
   services since no customer packets are modified or consumed or new
   packets introduced in the VPLS.  It is also slightly simpler to
   implement than PIM Proxy.  However for PIM Snooping to work
   correctly, it is a requirement that CE routers MUST disable Join
   suppression in the VPLS.

   Given that a large number of existing CE deployments do not support
   disabling of Join suppression and given the operational complexity
   for a provider to manage disabling of Join suppression in the VPLS,
   it becomes a difficult solution to deploy.  Another disadvantage of



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   PIM Snooping as a solution is that it does not scale as well as PIM
   Proxy.  If there are a large number of CEs in a VPLS, then every CE
   will see every other CE's Join/Prune messages.

   PIM Proxy on the PEs has the advantage that it does not require Join
   suppression to be disabled in the VPLS.  Multicast as a VPLS service
   can be very easily be provided without requiring any changes on the
   CE routers.  It also helps scale VPLS Multicast very well since the
   PEs intelligently forward only one Join/Prune message for a given
   flow and only to the upstream CE.

   PIM Proxy as a solution however loses the transparency argument since
   Join/Prunes could get modified or even consumed at a PE.  Also, new
   packets could get introduced in the VPLS.  However, this loss of
   transparency is limited to PIM Join/Prune packets.  It is in the
   interest of optimizing multicast in the VPLS and helping a VPLS
   network scale much better.  Data traffic will still be completely
   transparent.

2.5.1.  Differences between PIM Snooping and PIM Proxy

   For PIM-SM and PIM-BIDIR, a PIM Snooping/Proxy Switch only needs to
   examine PIM Hello and Join/Prune messages.  PIM Proxy for PIM-DM is
   for future study and is not currently specified in this draft.

   A proxy switch performs proxy only for the Join/Prune messages.  PIM
   Hello messages are snooped by both PIM Snooping and PIM Proxy
   switches.

   Details on the PIM Proxy solution are discussed in section
   Section 3.7.  This section is presented here to say that most of the
   procedures to follow (unless explicitly specified) are common to both
   PIM Snooping and PIM Proxy.  Differences between a PIM Snooping
   switch and a PIM Proxy switch can be summarized as the following:

















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   +------------------------------|--------------------------------+
   |     PIM Snooping             |       PIM Proxy                |
   +==============================|================================+
   | 1. PIM Snooping switches     | 1. PIM Proxy switches also     |
   |    snoop Hello and Join/Prune|    snoop PIM Hello messages    |
   |    messages while they are   |    while they are transparently|
   |    transparently flooded in  |    flooded in the VPLS. But    |
   |    the VPLS.                 |    they consume PIM Join/Prune |
   |                              |    messages and do not flood   |
   |                              |    them as is in the VPLS.     |
   +------------------------------|--------------------------------+
   | 2. PIM Snooping switches do  | 2. PIM Proxy switches may      |
   |    not originate any PIM     |    originate new or modified   |
   |    packets.                  |    Join/Prune packets.         |
   +------------------------------|--------------------------------+

   Other than the above simple differences, most of the procedures are
   common to PIM Snooping and PIM Proxy.  In the text to follow, we
   describe the procedures for PIM "Snooping".  Unless specifically
   stated otherwise, such procedures apply to PIM Proxy as well.

2.5.2.  PIM Control Message Latency

   A PIM Snooping or PIM Proxy switch snoops on PIM Hello packets while
   transparently flooding it in the VPLS.  As such there is no latency
   introduced by the VPLS in the delivery of PIM Hello packets to remote
   CEs in the VPLS.

   A PIM Proxy switch consumes PIM Join/Prune packets and generates
   proxy Join/Prune packets to be sent upstream.  This can result in
   additional latency for a downstream CE to receive multicast traffic
   after it has sent a Join.  When a downstream CE prunes a multicast
   stream, the traffic should stop flowing to the CE with no additional
   latency introduced by the VPLS.

   A PIM Snooping switch snoops on PIM Join/Prune packets while
   transparently flooding them in the VPLS.  There is no latency
   introduced by the VPLS in the delivery of PIM Join/Prune packets when
   PIM Snooping is employed.

2.5.3.  When to Snoop and When to Proxy

   Explicit Tracking (ET) is enabled in a VPLS when all PIM CE Routers
   in the VPLS advertise Tracking Support in their PIM Hello messages.
   If even one does not advertise Tracking Support, then all PIM CE
   routers disable ET in the VPLS.  When ET is enabled, it implies that
   Join Suppression is disabled and vice versa.




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   PIM Snooping PEs can determine if ET is enabled or disabled in a VPLS
   by examining PIM Hellos.  If ET is disabled, PIM Proxy MUST be used.
   If ET is enabled, either PIM Snooping or PIM Proxy can be used,
   unless the PIM control message latency due to proxy is a concern, in
   which case PIM Snooping SHOULD be used.


3.  PIM Snooping for VPLS

   IGMP snooping procedures described in [IGMP-SNOOP] provide efficient
   delivery of IP multicast traffic in a given VPLS service when end
   stations are connected to the VPLS.  However, when VPLS is offered as
   a WAN service it is likely that the CE devices are routers and would
   run PIM between them.  To provide efficient IP multicasting in such
   cases, it is necessary that the PE routers offering the VPLS service
   do PIM snooping.

   PIM is a multicast routing protocol, which runs exclusively between
   routers.  PIM shares many of the common characteristics of a routing
   protocol, such as discovery messages (e.g., neighbor discovery using
   Hello messages), topology information (e.g., multicast tree), and
   error detection and notification (e.g., dead timer and designated
   router election).  On the other hand, PIM does not participate in any
   kind of exchange of databases, as it uses the unicast routing table
   to provide reverse path information for building multicast trees.
   There are a few variants of PIM.  In [PIM-DM], multicast data is
   pushed towards the members similar to broadcast mechanism.  PIM-DM
   constructs a separate delivery tree for each multicast group.  As
   opposed to PIM-DM, other PIM flavors (PIM-SM [PIM-SM], PIM-SSM
   [PIM-SSM], and PIM-BIDIR [PIM-BIDIR]) invoke a pull methodology
   instead of push technique.

   PIM routers periodically exchange Hello messages to discover and
   maintain stateful sessions with neighbors.  After neighbors are
   discovered, PIM routers can signal their intentions to join or prune
   specific multicast groups.  This is accomplished by having downstream
   routers send an explicit Join/Prune message (for the sake of
   generalization, consider Graft messages for PIM-DM as Join messages)
   to the upstream routers.  The Join/Prune message can be group
   specific (*,G) or group and source specific (S,G).

   In PIM snooping, a PE snoops on the PIM message exchanged between
   routers, and builds its multicast states.

   Based on the multicast states, it forwards IP multicast traffic
   accordingly to avoid unnecessary flooding.

   In the following sub-sections, snooping mechanisms for each variety



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   of PIM are specified.

3.1.  General Rules for PIM Snooping in VPLS

   The following rules for the correct operation of PIM snooping MUST be
   followed.

   o  PIM messages and multicast data traffic forwarded by PEs MUST
      follow the split-horizon rule for mesh PWs.

   o  PIM snooping states in a PE MUST be per VPLS instance.

   o  PIM assert triggers MUST be preserved to the extent necessary to
      avoid sending duplicate traffic to the same PE (see
      Section 3.1.2).

3.1.1.  Snooping PIM Packets

   PIM-SM snooping PEs need to snoop on just the PIM Hello and PIM Join/
   Prune messages to build its multicast states.

   o  PIM-DM snooping PEs have to also snoop on PIM Graft and PIM State
      Refresh messages.

3.1.2.  Preserving Assert Trigger

   In PIM-SM/DM, there are scenarios where multiple routers could be
   forwarding the same multicast traffic on a LAN.  When this happens,
   using PIM Assert Election process by sending PIM Assert Messages,
   routers ensure that only the Assert Winner forwards traffic on the
   LAN.  The Assert Election is a data driven event and happens only if
   a router sees traffic on the interface to which it should be
   forwarding the traffic.  In the case of VPLS with snooping, two
   routers may forward the same flow at the same time but each copy may
   reach different set of PEs, and that is acceptable from the point of
   view of avoiding duplicate traffic.  If the two copies may reach the
   same PE then the sending routers must be able to see each other's
   traffic, in order to trigger Assert Election and stop duplicate
   traffic.

   To achieve that, PIM-SM Snooping MUST not only forward multicast
   traffic for an (S,G) on the ports on which they snooped Joins(S,G)/
   Joins(*,G), but also towards the upstream neighbor(s)).  In other
   words, the ports on which the upstream neighbors are learnt must be
   added to the outgoing port list along with the ports on which Joins
   are snooped.

   Similarly, PIM-DM Snooping SHOULD make sure that asserts can be



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   triggered (Section 3.6.3).

   The above logic needs to be facilitated without breaking VPLS Split
   Horizon Rules. i.e. traffic should not be forwarded on the port on
   which it was received, and traffic arriving on a PW MUST NOT be
   forwarded onto other PW(s).

3.2.  Discovering PIM Routers

   A PIM Snooping PE MUST snoop on PIM Hellos received on ACs and PWs.
   i.e. the PE transparently floods the PIM Hello while snooping on it.
   PIM Hellos are used by the snooping switch to discover PIM routers
   and their characteristics.

   For each neighbor discovered by a PE, it includes an entry in the PIM
   Neighbor Database with the following fields:

   o  Layer 2 encapsulation for the Router sending the PIM Hello.

   o  IP Address and address family of the Router sending the PIM Hello.

   o  Port (AC / PW) on which the PIM Hello was received.

   o  Hello TLVs

   The PE should be able to interpret and act on Hello TLVs currently
   defined in the PIM RFCs.  The TLVs of particular interest in this
   document are:

   o  Hello-Hold-Time

   o  Tracking Support

   o  DR Priority

   Please refer to [PIM-SM] for a list of the Hello TLVs.  When a PIM
   Hello is received, the PE MUST reset the neighbor-expiry- timer to
   Hello-Hold-Time.  If a PE does not receive a Hello message from a
   router within Hello-Hold-Time, the PE MUST remove that neighbor from
   its PIM Neighbor Database.  If a PE receives a Hello message from a
   router with Hello-Hold-Time value set to zero, the PE MUST remove
   that router from the PIM snooping state immediately.

   From the PIM Neighbor Database, a PE MUST be able to use the
   procedures defined in [PIM-SM] to identify the PIM Designated Router
   in the VPLS instance.  It should also be able to determine if
   Tracking Support is active in the VPLS instance.




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3.3.  PIM-SM and PIM-SSM

   The key characteristic of PIM-SM and PIM-SSM is explicit join
   behavior.  In this model, multicast traffic is only forwarded to
   locations that specifically request it.  The root node of a tree is
   the Rendezvous Point (RP) in case of a shared tree (PIM-SM only) or
   the first hop router that is directly connected to the multicast
   source in the case of a shortest path tree.  All the procedures
   described in this section apply to both PIM-SM and PIM-SSM, except
   for the fact that there is no (*,G) state in PIM-SSM.

3.3.1.  Building PIM-SM Snooping States

   PIM-SM and PIM-SSM Snooping states are built by snooping on the PIM-
   SM Join/Prune messages received on AC/PWs.

   The downstream state machine of a PIM-SM snooping switch very closely
   resembles the downstream state machine of PIM-SM routers.  The
   downstream state consists of:

   Per downstream (Port, *, G):

   o  DownstreamJPState: One of { "NoInfo" (NI), "Join" (J), "Prune
      Pending" (PP) }

   Per downstream (Port, *, G, N):

   o  Prune Pending Timer (PPT(N))

   o  Join Expiry Timer (ET(N))

   Per downstream (Port, S, G):

   o  DownstreamJPState: One of { "NoInfo" (NI), "Join" (J), "Prune
      Pending" (PP) }

   Per downstream (Port, S, G, N):

   o  Prune Pending Timer (PPT(N))

   o  Join Expiry Timer (ET(N))

   Per downstream (Port, S, G, rpt):

   o  DownstreamJPRptState: One of { "NoInfo" (NI), "Pruned" (P), "Prune
      Pending" (PP) }

   Per downstream (Port, S, G, rpt, N):



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   o  Prune Pending Timer (PPT(N))

   o  Join Expiry Timer (ET(N))

   Where S is the address of the multicast source, G is the Group
   address and N is the upstream neighbor field in the Join/Prune
   message.  Notice that unlike on PIM-SM routers where PPT and ET are
   per (Interface, S, G), PIM Snooping switches have to maintain PPT and
   ET per (Port, S, G, N).  The reasons for this are explained in
   section Section 3.3.2

   Apart from the above states, we define the following state
   summarization macros.

   UpstreamNeighbors(*,G): If there is one or more Join(*,G) received on
   any port with upstream neighbor N and ET(N) is active, then N is
   added to UpstreamNeighbors(*,G).  This set is used to determine if a
   Join(*,G) or a Prune(*,G) with upstream neighbor N needs to be sent
   upstream.

   UpstreamNeighbors(S,G): If there is one or more Join(S,G) received on
   any port with upstream neighbor N and ET(N) is active, then N is
   added to UpstreamNeighbors(S,G).  This set is used to determine if a
   Join(S,G) or a Prune(S,G) with upstream neighbor N needs to be sent
   upstream.

   UpstreamPorts(*,G): This is the set of all Rport(N) ports where N is
   in the set UpstreamNeighbors(*,G).  Multicast Streams forwarded using
   a (*,G) match MUST be forwarded to these ports in addition to
   downstream ports.  So UpstreamPorts(*,G) MUST be added to
   OutgoingPortList(*,G).

   UpstreamPorts(S,G): This is the set of all Rport(N) ports where N is
   in the set UpstreamNeighbors(S,G).  UpstreamPorts(S,G) MUST be added
   to OutgoingPortList(S,G).

   InheritedUpstreamPorts(S,G): This is the union of UpstreamPorts(S,G)
   and UpstreamPorts(*,G).

   UpstreamPorts(S,G,rpt): If PruneDesired(S,G,rpt) becomes true, then
   this set is set to UpstreamPorts(*,G).  Otherwise, this set is empty.
   UpstreamPorts(*,G) (-) UpstreamPorts(S,G,rpt) MUST be added to
   OutgoingPortList(S,G).

   UpstreamPorts(G): This set is the union of all the UpstreamPorts(S,G)
   and UpstreamPorts(*,G) for a given G. Proxy (S,G) Join/Prune and
   (*,G) Join/Prune messages MUST be sent to a subset of
   UpstreamPorts(G) as specified in section Section 3.3.6.1.



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   PWPorts: This is the set of all PWs.

   OutgoingPortList(*,G): This is the set of all ports to which traffic
   needs to be forwarded on a (*,G) match.

   OutgoingPortList(S,G): This is the set of all ports to which traffic
   needs to be forwarded on an (S,G) match.

   See section Section 3.9 on Data Forwarding Rules for the
   specification on how OutgoingPortList is calculated.

   NumETsActive(Port,*,G): Number of (Port,*,G,N) entries that have
   Expiry Timer running.  This macro keeps track of the number of
   Join(*,G)s that are received on this Port with different upstream
   neighbors.

   NumETsActive(Port,S,G): Number of (Port,S,G,N) entries that have
   Expiry Timer running.  This macro keeps track of the number of
   Join(*,G)s that are received on this Port with different upstream
   neighbors.

   RpfVectorTlvs(*,G): RPF Vectors [RPF-VECTOR] are TLVs that may be
   present in received Join(*,G) messages.  If present, they must be
   copied to RpfVectorTlvs(*,G).

   RpfVectorTlvs(S,G): RPF Vectors [RPF-VECTOR] are TLVs that may be
   present in received Join(S,G) messages.  If present, they must be
   copied to RpfVectorTlvs(S,G).

   Since there are a few differences between the downstream state
   machines of PIM-SM Routers and PIM-SM snooping switches, we specify
   the details of the downstream state machine of PIM-SM snooping
   switches at the risk of repeating most of the text documented in
   [PIM-SM].

3.3.2.  Explanation for per (S,G,N) states

   In PIM Routing protocols, states are built per (S,G).  On a router,
   an (S,G) has only one RPF-Neighbor.  However, a PIM Snooping switch
   does not have the Layer 3 routing information available to the
   routers in order to determine the RPF-Neighbor for a multicast flow.
   It merely discovers it by snooping the Join/Prune message.  A PE
   could have snooped on two or more different Join/Prune messages for
   the same (S,G) that could have carried different Upstream-Neighbor
   fields.  This could happen during transient network conditions or due
   to dual- homed sources.  A PE cannot make assumptions on which one to
   pick, but instead must facilitate the CE routers decide which
   Upstream Neighbor gets elected the RPF-Neighbor.  And for this



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   purpose, the PE will have to track downstream and upstream Join/Prune
   states per (S,G,N).

3.3.3.  Receiving (*,G) PIM-SM Join/Prune Messages

   A Join(*,G) or Prune(*,G) is considered "received" if the following
   conditions are met:

   o  The port on which it arrived is not Rport(N) where N is the
      upstream-neighbor N of the Join/Prune(*,G), or,

   o  if both RPort(N) and the arrival port are PWs, then there exists
      at least one other (*,G,Nx) or (Sx,G,Nx) state with an AC
      UpstreamPort.

   For simplicity, the case where both RPort(N) and the arrival port are
   PWs is referred to as PW-only Join/Prune in this document.  The PW-
   only Join/Prune handling is so that the RPort(N) PW can be added to
   the related forwarding entries' OutgoingPortList to trigger Assert,
   but that is only needed for those states with AC UpstreamPort.  Note
   that in PW-only case, it is ok for the arrival port and RPort(N) to
   be the same.  See Appendix Appendix B for examples.

   When a router receives a Join(*,G) or a Prune(*,G) with upstream
   neighbor N, it must process the message as defined in the state
   machine below.  Note that the macro computations of the various
   macros resulting from this state machine transition is exactly as
   specified in the PIM-SM RFC [PIM-SM].

   We define the following per-port (*,G,N) macro to help with the state
   machine below.




















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   Figure 1 : Downstream per-port (*,G) state machine in tabular form
   +---------------++----------------------------------------+
   |               ||          Previous State                |
   |               ++------------+--------------+------------+
   | Event         ||NoInfo (NI) | Join (J)     | Prune-Pend |
   +---------------++------------+--------------+------------+
   | Receive       ||-> J state  | -> J state   | -> J state |
   | Join(*,G)     || Action     | Action       | Action     |
   |               || RxJoin(N)  | RxJoin(N)    | RxJoin(N)  |
   +---------------++------------+--------------+------------+
   |Receive        || -          | -> PP state  | -> PP state|
   |Prune(*,G) and ||            | Start PPT(N) |            |
   |NumETsActive<=1||            |              |            |
   +---------------++------------+--------------+------------+
   |Receive        || -          | -> J state   | -          |
   |Prune(*,G) and ||            | Start PPT(N) |            |
   |NumETsActive>1 ||            |              |            |
   +---------------++------------+--------------+------------+
   |PPT(N) expires || -          | -> J state   | -> NI state|
   |               ||            | Action       | Action     |
   |               ||            | PPTExpiry(N) |PPTExpiry(N)|
   +---------------++------------+--------------+------------+
   |ET(N) expires  || -          | -> NI state  | -> NI state|
   |and            ||            | Action       | Action     |
   |NumETsActive<=1||            | ETExpiry(N)  | ETExpiry(N)|
   +---------------++------------+--------------+------------+
   |ET(N) expires  || -          | -> J state   | -> NI state|
   |and            ||            | Action       | Action     |
   |NumETsActive>1 ||            | ETExpiry(N)  | ETExpiry(N)|
   +---------------++------------+--------------+------------+

   Action RxJoin(N):

      If ET(N) is not already running, then start ET(N).  Otherwise
      restart ET(N).  If N is not already in UpstreamNeighbors(*,G),
      then add N to UpstreamNeighbors(*,G) and trigger a Join(*,G) with
      upstream neighbor N to be forwarded upstream.  If there are RPF
      Vector TLVs in the received (*,G) message and if they are
      different from the recorded RpfVectorTlvs(*,G), then copy them
      into RpfVectorTlvs(*,G).

   Action PPTExpiry(N):

      Same as Action ETExpiry(N) below, plus Send a Prune-Echo(*,G) with
      upstream-neighbor N on the downstream port.

   Action ETExpiry(N):




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      Disable timers ET(N) and PPT(N).  Delete neighbor state
      (Port,*,G,N).  If there are no other (Port,*,G) states with
      NumETsActive(Port,*,G) > 0, transition DownstreamJPState to
      NoInfo.  If there are no other (Port,*,G,N) state (different ports
      but for the same N), remove N from UpstreamPorts(*,G) - this also
      serves as a trigger for US FSM (JoinDesired(*,G,N) becomes FALSE).

3.3.4.  Receiving (S,G) PIM-SM Join/Prune Messages

   A Join(S,G) or Prune(S,G) is considered "received" if the following
   conditions are met:

   o  The port on which it arrived is not Rport(N) where N is the
      upstream-neighbor N of the Join/Prune(S,G), or,

   o  if both RPort(N) and the arrival port are PWs, then there exists
      at least one other (*,G,Nx) or (S,G,Nx) state with an AC
      UpstreamPort.

   For simplicity, the case where both RPort(N) and the arrival port are
   PWs is referred to as PW-only Join/Prune in this document.  The PW-
   only Join/Prune handling is so that the RPort(N) PW can be added to
   the related forwarding entries' OutgoingPortList to trigger Assert,
   but that is only needed for those states with AC UpstreamPort.  See
   Appendix Appendix B for examples.

   When a router receives a Join(S,G) or a Prune(S,G) with upstream
   neighbor N, it must process the message as defined in the state
   machine below.  Note that the macro computations of the various
   macros resulting from this state machine transition is exactly as
   specified in the PIM-SM RFC [PIM-SM].




















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   Figure 2: Downstream per-port (S,G) state machine in tabular form
   +---------------++----------------------------------------+
   |               ||              Previous State            |
   |               ++------------+--------------+------------+
   |   Event       ||NoInfo (NI) | Join (J)     | Prune-Pend |
   +---------------++------------+--------------+------------+
   | Receive       ||-> J state  | -> J state   | -> J state |
   | Join(S,G)     || Action     | Action       | Action     |
   |               || RxJoin(N)  | RxJoin(N)    | RxJoin(N)  |
   +---------------++------------+--------------+------------+
   |Receive        || -          | -> PP state  | -> PP state|
   |Prune (S,G) and||            | Start PPT(N) |            |
   |NumETsActive<=1||            |              |            |
   +---------------++------------+--------------+------------+
   |Receive        || -          | -> J state   | -          |
   |Prune(S,G) and ||            | Start PPT(N) |            |
    NumETsActive>1 ||            |              |            |
   +---------------++------------+--------------+------------+
   |PPT(N) expires || -          | -> J state   | -> NI state|
   |               ||            | Action       | Action     |
   |               ||            | PPTExpiry(N) |PPTExpiry(N)|
   +---------------++------------+--------------+------------+
   |ET(N) expires  || -          | -> NI state  | -> NI state|
   |and            ||            | Action       | Action     |
   |NumETsActive<=1||            | ETExpiry(N)  | ETExpiry(N)|
   +---------------++------------+--------------+------------+
   |ET(N) expires  || -          | -> J state   | -> NI state|
   |and            ||            | Action       | Action     |
   |NumETsActive>1 ||            | ETExpiry(N)  | ETExpiry(N)|
   +---------------++------------+--------------+------------+

   Action RxJoin(N):

      If ET(N) is not already running, then start ET(N).  Otherwise,
      restart ET(N).

      If N is not already in UpstreamNeighbors(S,G), then add N to
      UpstreamNeighbors(S,G) and trigger a Join(S,G) with upstream
      neighbor N to be forwarded upstream.  If there are RPF Vector TLVs
      in the received (S,G) message and if they are different from the
      recorded RpfVectorTlvs(S,G), then copy them into
      RpfVectorTlvs(S,G).

   Action PPTExpiry(N):

      Same as Action ETExpiry(N) below, plus Send a Prune-Echo(S,G) with
      upstream-neighbor N on the downstream port.




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   Action ETExpiry(N):

      Disable timers ET(N) and PPT(N).  Delete neighbor state
      (Port,S,G,N).  If there are no other (Port,S,G) states with
      NumETsActive(Port,S,G) > 0, transition DownstreamJPState to
      NoInfo.  If there are no other (Port,S,G,N) state (different ports
      but for the same N), remove N from UpstreamPorts(S,G) - this also
      serves as a trigger for US FSM (JoinDesired(S,G,N) becomes FALSE).

3.3.5.  Receiving (S,G,rpt) Join/Prune Messages

   A Join(S,G,rpt) or Prune(S,G,rpt) is "received" when the port on
   which it was received is not also the port on which the upstream-
   neighbor N of the Join/Prune(S,G,rpt) was learnt.

   While it is important to ensure that the (S,G) and (*,G) state
   machines allow for handling per (S,G,N) states, it is not as
   important for (S,G,rpt) states.  It suffices to say that the
   downstream (S,G,rpt) state machine is the same as what is defined in
   section 4.5.4 of the PIM-SM RFC [PIM-SM].

3.3.6.  Sending Join/Prune Messages Upstream

   This section applies only to a PIM Proxy Switch and not to a PIM
   Snooping Switch.

   A PIM Proxy PE MUST implement the Upstream FSM for which the
   procedures are similar to what is defined in section 4.5.6 of
   [PIM-SM].  Similar to Downstream FSM described above, the Upstream
   FSM is also per Upstream Neighbor.

   For the purposes of the Upstream FSM, a Join or Prune message with
   upstream neighbor N is "seen" on a PIM Snooping switch if the port on
   which the message was received is also Rport(N), and the port is an
   AC.  The AC requirement is needed because a Join received on the
   Rport(N) PW must not suppress this PE's Join on that PW.

   In order to correctly facilitate assert among the CE routers, such
   Join/Prunes need to sent not only towards the upstream neighbor, but
   also on certain PWs as described below.

   If RpfVectorTlvs(*,G) is not empty, then it must be encoded in a
   Join(*,G) message sent upstream.

   If RpfVectorTlvs(S,G) is not empty, then it must be encoded in a
   Join(S,G) message sent upstream.





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3.3.6.1.  Where to send Join/Prune messages

   The following rules apply, to both refresh and triggered (S,G)/(*,G)
   Join/Prune messages.

   o  The upstream neighbor field N in the Join/Prune to be sent is set
      to the N in the corresponding Upstream FSM.

   o  if Rport(N) is an AC, send the message to Rport(N).

   o  Additionally, if OutgoingPortList(X,G,N) contains at lease one AC,
      then the message MUST be sent to at least all the PWs in
      UpstreamPorts(G) (for (*,G)) or InheritedUpstreamPorts(S,G) (for
      (S,G)).  Alternatively, the message MAY be sent to all PWs.

   Sending to a subset of PWs as described above guarantees that if
   traffic (of the same flow) from two upstream routers were to reach
   this PE, then the two routers will receive from each other,
   triggering assert.

   Sending to all PWs guarantees that if two upstream routers both send
   traffic for the same flow (even if it is to different set of
   downstream PEs), then they'll receive from each other, triggering
   assert.

3.4.  Bidirectional-PIM (PIM-BIDIR)

   PIM-BIDIR is a variation of PIM-SM.  The main differences between
   PIM-SM and Bidirectional-PIM are as follows:

   o  There are no source-based trees, and source-specific multicast is
      not supported (i.e., no (S,G) states) in PIM- BIDIR.

   o  Multicast traffic can flow up the shared tree in PIM-BIDIR.

   o  To avoid forwarding loops, one router on each link is elected as
      the Designated Forwarder (DF) for each RP in PIM-BIDIR.

   The main advantage of PIM-BIDIR is that it scales well for many-to-
   many applications.  However, the lack of source-based trees means
   that multicast traffic is forced to remain on the shared tree.

   As described in [PIM-BIDIR], parts of a PIM-BIDIR enabled network may
   forward traffic without exchanging Join/Prune messages, for instance
   between DF's and the RPL.

   As the described procedures for Pim snooping rely on the presence of
   Join/Prune messages, enabling Pim snooping on PIM-BIDIR networks



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   could break the PIM-BIDIR functionality.  Deploying Pim snooping on
   PIM-BIDIR enabled networks will require some further study, some
   thoughts are gathered in Appendix A.

3.5.  Interaction with IGMP Snooping

   Whenever IGMP Snooping is enabled in conjunction with PIM Snooping in
   the same VPLS instance the switch SHOULD follow these rules:

   o  To maintain the list of multicast routers and ports on which they
      are attached, the switch SHOULD NOT use the rules as described in
      section 2.1.1.(1) of RFC4541 [IGMP-SNOOP] but SHOULD rely on the
      neighbors discovered by PIM Snooping .  This list SHOULD then be
      used to apply the forwarding rule as described in 2.1.1.(1) of
      RFC4541 [IGMP-SNOOP].

   o  If the switch supports proxy-reporting, as described in section
      2.1.1.(2) of RFC4541 [IGMP-SNOOP], all IGMP membership information
      learned on a port to which a PIM neighbor is attached SHOULD NOT
      be included in the summarized upstream report.

3.6.  PIM-DM

   The characteristics of PIM-DM is flood and prune behavior.  Shortest
   path trees are built as a multicast source starts transmitting.

3.6.1.  Building PIM-DM Snooping States

   PIM-DM Snooping states are built by snooping on the PIM-DM Join,
   Prune, Graft and State Refresh messages received on AC/PWs and State-
   Refresh Messages sent on AC/PWs.  By snooping on these PIM-DM
   messages, a PE builds the following states per (S,G,N) where S is the
   address of the multicast source, G is the Group address and N is the
   upstream neighbor to which Prunes/Grafts are sent by downstream CEs:

   Per PIM (S,G,N):

      Port PIM (S,G,N) Prune State:



      *  DownstreamPState(S,G,N,Port): One of {"NoInfo" (NI), "Pruned"
         (P), "PrunePending" (PP)}

      *  Prune Pending Timer (PPT)

      *  Prune Timer (PT)




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      *  Upstream Port (valid if the PIM(S,G,N) Prune State is
         "Pruned").

3.6.2.  PIM-DM Downstream Per-Port PIM(S,G,N) State Machine

   The downstream per-port PIM(S,G,N) state machine is as defined in
   section 4.4.2 of [PIM-DM] with a few changes relevant to PIM
   Snooping.  When reading section 4.4.2 of [PIM-DM] for the purposes of
   PIM-Snooping please be aware that the downstream states are built per
   (S, G, N, Downstream-Port} in PIM-Snooping and not per {Downstream-
   Interface, S, G} as in a PIM-DM router.  As noted in the previous
   section Section 3.6.1, the states (DownstreamPState) and timers (PPT
   and PT) are per (S,G,N,P).

3.6.3.  Triggering ASSERT election in PIM-DM

   Since PIM-DM is a flood-and-prune protocol, traffic is flooded to all
   routers unless explicitly pruned.  Since PIM-DM routers do not prune
   on non-RPF interfaces, PEs should typically not receive Prunes on
   Rport(RPF-neighbor).  So the asserting routers should typically be in
   pim_oiflist(S,G).  In most cases, assert election should occur
   naturally without any special handling since data traffic will be
   forwarded to the asserting routers.

   However, there are some scenarios where a prune might be received on
   a port which is also an upstream port (UP).  If we prune the port
   from pim_oiflist(S,G), then it would not be possible for the
   asserting routers to determine if traffic arrived on their downstream
   port.  This can be fixed by adding pim_iifs(S,G) to pim_oiflist(S,G)
   so that data traffic flows to the UP ports.

3.7.  PIM Proxy

   As noted earlier, PIM Snooping will work correctly only if Join
   Suppression is disabled in the VPLS.  If Join Suppression is enabled
   in the VPLS, then PEs MUST do PIM Proxy for VPLS Multicast to work
   correctly.

   A PIM Proxy switch behaves like a PIM Router by doing most of the
   functionality of a PIM Router.  The complexity however is much lesser
   on a switch since many of the issues that a PIM Router has to deal
   with are not relevant on a switch.  A PIM Router needs to be able to
   build and maintain RP-Sets.  They also have to deal with the Register
   and Assert State Machines.  There are other complexities for a PIM
   Router resulting from inter-domain multicast.  A PIM Snooping or PIM
   Proxy switch can be agnostic of all of this.  All that a PIM Proxy
   switch cares about is building multicast states using PIM Hellos and
   PIM Join/Prune message.  As such it's complexity is greatly reduced.



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   Other than the procedures defined here, the rest of the procedures
   that apply to PIM Snooping apply to PIM Proxy as well.

3.7.1.  Downstream PIM Proxy behavior

   Only PIM Join/Prune messages are proxied.  Hellos MUST be snooped
   while being flooded in the VPLS. i.e.  PIM Hellos MUST NOT be
   consumed at a PE and regenerated.

   All other PIM packet types are flooded in the VPLS without any
   processing.

   Performing only proxy of Join/Prune messages keeps the switch
   behavior very similar to that of a PIM router without introducing too
   much additional complexity.  It keeps the PIM Proxy solution fairly
   simple.  Since Join/Prunes are forwarded by a PE along the slow-path
   and all other PIM packet types are forwarded along the fast-path, it
   is very likely that packets forwarded along the fast-path will arrive
   "ahead" of Join/Prune packets at a CE router (note the stress on the
   fact that fast-path messages will never arrive after Join/Prunes).
   Of particular importance are Hello packets sent along the fast-path.
   We can construct a variety of scenarios resulting in out of order
   delivery of Hellos and Join/Prune messages.  However, there should be
   no deviation from normal expected behavior observed at the CE router
   receiving these messages out of order.

   The other option for a PIM Proxy solution is to proxy both Hello and
   Join/Prune messages that a PE is interested in building states for.
   If Hellos are being proxied, then it becomes necessary that the PE
   proxy all other PIM packet types also.  Because if Hellos are
   received after other packet types are received at a CE router, then
   bad things will happen.  That means every PIM packet has to be sent
   along the slow-path.  This greatly increases the complexity on the CE
   router, it is very compute intensive and does not scale well.  Also,
   proxying Hellos will result in added latency to delivery of Hello
   messages to a CE and that affects multicast convergence in the VPLS.

3.7.2.  Upstream PIM Proxy behavior

   Since a PIM Proxy switch consumes Join/Prune messages, it must also
   originate PIM Join/Prune messages to be sent upstream.  On ACs, both
   triggered and refresh Join/Prunes are forwarded as PIM packets.

3.7.3.  Source IP Address in Proxy PIM Join/Prune Packets

   The source IP address in PIM packets sent upstream SHOULD be the
   address of a PIM downstream neighbor in the corresponding join/prune
   state.  The address picked MUST NOT be the upstream neighbor field to



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   be encoded in the packet.  The layer 2 encapsulation for the selected
   source IP address MUST be the encapsulation recorded in the PIM
   Neighbor database for that IP address.

   If Explicit Tracking (ET) is disabled in the VPLS, then it does not
   matter what Source IP Address is picked in the packets sent upstream
   as long as we adhere to the rule in the previous paragraph.

   If ET is enabled, it means that a CE router is interested in tracking
   every CE that wishes to join a stream.  If a PE determines that ET is
   enabled, then it SHOULD use PIM Snooping procedures instead of PIM
   Proxy.

3.8.  Directly Connected Multicast Source

   If there is a source in the CE network that connects directly into
   the VPLS instance, then multicast traffic from that source MUST be
   sent to all PIM routers on the VPLS instance apart from the igmp
   receivers in the VPLS.  If there is already (S,G) or (*,G) snooping
   state that is formed on any PE, this will not happen per the current
   forwarding rules and guidelines.  So, in order to determine if
   traffic needs to be flooded to all routers, a PE must be able to
   determine if the traffic came from a host on that LAN.  There are
   three ways to address this problem:

   o  The PE would have to do ARP snooping to determine if a source is
      directly connected.

   o  Another option is to have configuration on all PEs to say there
      are CE sources that are directly connected to the VPLS instance
      and disallow snooping for the groups for which the source is going
      to send traffic.  This way traffic from that source to those
      groups will always be flooded within the provider network.

   o  A third option is to require that sources of CE multicast routers
      must appear behind a router.

3.9.  Data Forwarding Rules

   First we define the rules that are common to PIM-SM, PIM-BIDIR and
   PIM-DM PEs.  Forwarding rules for each protocol type is specified in
   the sub-sections.

   If there is no matching forwarding state, then the PE MAY either
   discard the packet or send it towards all the snooped PIM CE routers
   or to a configured set of ports.  How this is determined is outside
   the scope of this document.




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   The following general rules MUST be followed when forwarding
   multicast traffic in a VPLS:

   o  Traffic arriving on a port MUST NOT be forwarded back onto the
      same port.

   o  Due to VPLS Split-Horizon rules, traffic ingressing on a PW MUST
      NOT be forwarded to any other PW.

3.9.1.  PIM-SM Data Forwarding Rules

   Per the rules in [PIM-SM] and per the additional rules specified in
   this document,

   OutgoingPortList(*,G) = immediate_olist(*,G) (+)
                           UpstreamPorts(*,G) (+)
                           Rport(PimDR)

   OutgoingPortList(S,G) = inherited_olist(S,G) (+)
                           UpstreamPorts(S,G) (+)
                           (UpstreamPorts(*,G) (-)
                            UpstreamPorts(S,G,rpt)) (+)
                           Rport(PimDR)

   [PIM-SM]specifies how immediate_olist(*,G) and inherited_olist(S,G)
   are built.  PimDR is the IP address of the PIM DR in the VPLS.

   The PIM-SM Snooping forwarding rules are defined below in pseudocode:























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   BEGIN
       iif is the incoming port of the multicast packet.
       S is the Source IP Address of the multicast packet.
       G is the Destination IP Address of the multicast packet.

       If there is (S,G) state on the PE
       Then
           OutgoingPortList = OutgoingPortList(S,G)
       Else if there is (*,G) state on the PE
       Then
           OutgoingPortList = OutgoingPortList(*,G)
       Else
           OutgoingPortList = UserDefinedPortList
       Endif

       If iif is an AC
       Then
           OutgoingPortList = OutgoingPortList (-) iif
       Else
           ## iif is a PW
           OutgoingPortList = OutgoingPortList (-) PWPorts
       Endif

       Forward the packet to OutgoingPortList.
   END

   First if there is (S,G) state on the PE, then the set of outgoing
   ports is OutgoingPortList(S,G).

   Otherwise if there is (*,G) state on the PE, the set of outgoing
   ports is OutgoingPortList(*,G).

   The packet is forwarded to the selected set of outgoing ports while
   observing the general rules above in section Section 3.9

3.9.2.  PIM-BIDIR Data Forwarding Rules

   The PIM-BIDIR Snooping forwarding rules are defined below in
   pseudocode:












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   BEGIN
       iif is the incoming port of the multicast packet.
       G is the Destination IP Address of the multicast packet.

       If there is forwarding state for G
       Then
           OutgoingPortList = olist(G)
       Else
           OutgoingPortList = UserDefinedPortList
       Endif

       If iif is an AC
       Then
           OutgoingPortList = OutgoingPortList (-) iif
       Else
           ## iif is a PW
           OutgoingPortList = OutgoingPortList (-) PWPorts
       Endif

       Forward the packet to OutgoingPortList.
   END

   If there is forwarding state for G, then forward the packet to
   olist(G) while observing the general rules above in section
   Section 3.9

   [PIM-BIDIR] specifies how olist(G) is contructed.

3.9.3.  PIM-DM Data Forwarding Rules

   The PIM-DM Snooping data forwarding rules are defined below in
   pseudocode:



















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   BEGIN
       iif is the incoming port of the multicast packet.
       S is the Source IP Address of the multicast packet.
       G is the Destination IP Address of the multicast packet.

       If there is (S,G) state on the PE
       Then
           OutgoingPortList = olist(S,G)
       Else
           OutgoingPortList = UserDefinedPortList
       Endif

       If iif is an AC
       Then
           OutgoingPortList = OutgoingPortList (-) iif
       Else
           ## iif is a PW
           OutgoingPortList = OutgoingPortList (-) PWPorts
       Endif

       Forward the packet to OutgoingPortList.
   END

   If there is forwarding state for (S,G), then forward the packet to
   olist(S,G) while observing the general rules above in section
   Section 3.9

   [PIM-DM] specifies how olist(S,G) is contructed.


4.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.


5.  Security Considerations

   Security considerations provided in VPLS solution documents (i.e.,
   [VPLS-LDP] and [VPLS-BGP]) apply to this document as well.


6.  Contributers

   Karl (Xiangrong) Cai and Princy Elizabeth made significant
   contributions to bring the specification to its current state,



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   especially in the area of Join forwarding rules.

   Yetik Serbest, Ray Qiu, Suresh Boddapati co-authored earlier
   versions.


7.  Acknowledgements

   Many members of the L2VPN and PIM working groups have contributed to
   and provided valuable comments and feedback to this draft, including
   Vach Kompella, Shane Amante, Sunil Khandekar, Rob Nath, Marc Lassere,
   Yuji Kamite, Yiqun Cai, Ali Sajassi, Jozef Raets, Himanshu Shah
   (Ciena), Himanshu Shah (Alcatel-Lucent).


8.  References

8.1.  Normative References

   [PIM-BIDIR]
              Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, 2007.

   [PIM-DM]   Adams, A., Nicholas, J., and W. Siadak, "Protocol
              Independent Multicast Version 2 - Dense Mode
              Specification", RFC 3973, 2005.

   [PIM-SM]   Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast- Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, 2006.

   [PIM-SSM]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, 2006.

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

   [RPF-VECTOR]
              Wijnands, I., Boers, A., and E. Rosen, "The Reverse Path
              Forwarding (RPF) Vector TLV", RFC 5496, 2009.

8.2.  Informative References

   [IGMP-SNOOP]
              Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for IGMP and MLD Snooping Switches",
              RFC 4541, 2006.



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   [VPLS-BGP]
              Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
              using BGP for Auto-Discovery and Signaling", RFC 4761,
              2007.

   [VPLS-LDP]
              Lasserre, M. and V. Kompella, "Virtual Private LAN
              Services using LDP Signaling", RFC 4762, 2007.

   [VPLS-MCAST-REQ]
              Kamite, Y., Wada, Y., Serbest, Y., Morin, T., and L. Fang,
              "Requirements for Multicast Support in Virtual Private LAN
              Services", RFC 5501, 2009.

   [VPLS-MCAST-TREES]
              Aggarwal, R., Kamite, Y., Fang, L., and Y. Rekhter,
              "Multicast in VPLS",  draft-ietf-l2vpn-vpls-mcast-10, Work
              in Progress.


Appendix A.  PIM-BIDIR Thoughts

   This section describes some guidelines that may be used to preserve
   PIM-BIDIR functionality in combination with Pim Snooping.

   In order to preserve PIM-BIDIR Pim snooping routers need to set up
   forwarding states so that :

   o  on the RPL all traffic is forwarded to all Rport(N)

   o  on any other interface traffic is always forwarded to the DF

   The information needed to setup these states may be obtained by :

   o  determining the mapping between group(range) and RP

   o  snooping and storing DF election information

   o  determining where the RPL is, this could be achieved by static
      configuration, or by combining the information mentioned in
      previous bullets.


Appendix B.  Example Network Scenario

   Let us consider the scenario in Figure 3.





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   An Example Network for Triggering Assert
    /----\   +------+ AC5 +------+             +------+ AC3 +------+   /----\
    | S2 |---| CE5  |-----|  PE4 |-------------|  PE2 |-----| CE3  |---| S3 |
    \----/   +------+     |      |\   PW24    /|      |     +------+   \----/
                          +------+ \         / +------+         |
                              |     \       /     |             |
                              |      \     /      |             |
                              |   PW34\   /PW12   |             |
                              |        \ /        |           /---\
                         PW14 |         /         |PW23       | S |
                              |        / \        |           \---/
                              |       /   \       |             |
                              |      /     \      |             |
                              |     /       \     |             |
                          +------+ /         \ +------+         |
             +------+     |  PE1 |/   PW13    \|  PE3 |     +------+
             | CE1  |-----|      |-------------|      |-----| CE4  |
             +------+ AC1 +------+             +------+ AC4 +------+
                              |
                              |AC2
                          +------+
                          | CE2  |
                          +------+

   In the examples below, JT(Port,S,G,N) is the downstream Join Expiry
   Timer on the specified Port for the (S,G) with upstream neighbor N.

B.1.  Pim Snooping Example

   In the network depicted in Figure 3, S is the source of a multicast
   stream (S,G).  CE1 and CE2 both have two ECMP routes to reach the
   source.
    1. CE1 Sends a Join(S,G) with Upstream Neighbor(S,G) = CE3.
    2. PE1 snoops on the Join(S,G) while flooding it in the VPLS. PE2
       and PE3 also snoop on the Join(S,G) while flooding it in the
       VPLS.

       The resulting states at the PEs is as follows:

        At PE1:
              JT(AC1,S,G,CE3)        = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { PW12 }
              OutgoingPortList(S,G)  = { AC1, PW12 }

          At PE2:
              JT(PW12,S,G,CE3)       = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE3 }



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              UpstreamPorts(S,G)     = { AC3 }
              OutgoingPortList(S,G)  = { PW12, AC3 }

          At PE3:
              PE3 doesn't create a forwarding state for (S,G) because
              the Join(S,G) was received on a PW and the Upstream RPort
              is a PW too. <<<<<

    3. The multicast stream (S,G) flows along CE3 -> PE2 -> PE1 -> CE1
    4. Now CE2 sends a Join(S,G) with Upstream Neighbor(S,G) = CE4.
    5. All PEs snoop on the Join(S,G).

       The resulting states at the PEs:

          At PE1:
              JT(AC1,S,G,CE3)        = active
              JT(AC2,S,G,CE4)        = JP_HoldTime.
              UpstreamNeighbors(S,G) = { CE3, CE4 }
              UpstreamPorts(S,G)     = { PW12, PW13 }
              OutgoingPortList(S,G)  = { AC1, PW12, AC2, PW13 }

          At PE2:  Note: Since PE2 already has (S,G) state, it does not
                   ignore the Join(S,G) even though it received the
                   Join(S,G) on a PW and the Upstream Rport is a PW. <<<<<<
              JT(PW12,S,G,CE4)       = JP_HoldTime
              JT(PW12,S,G,CE3)       = active
              UpstreamNeighbors(S,G) = { CE3, CE4 }
              UpstreamPorts(S,G)     = { AC3, PW23 }
              OutgoingPortList(S,G)  = { PW12, AC3, PW23 }

          At PE3:
              JT(PW13,S,G,CE4)       = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE4 }
              UpstreamPorts(S,G)     = { AC4 }
              OutgoingPortList(S,G)  = { PW13, AC4 }

    6. The multicast stream (S,G) flows into the VPLS from the two CEs
       CE3 and CE4. PE2 forwards the stream received from CE3 to PW23
       and PE3 forwards the stream to AC4. This facilitates the CE
       routers to trigger assert election. Let us say CE3 becomes the
       assert winner.
    7. CE3 sends an Assert message to the VPLS. The PEs flood the
       Assert message without examining it.
    8. CE4 stops sending the multicast stream to the VPLS.
    9. CE2 notices an RPF change due to Assert and sends a Prune(S,G)
       with Upstream Neighbor = CE4. CE2 also sends a Join(S,G) with
       Upstream Neighbor = CE3.
   10. All the PEs start a prune-pend timer on the ports on which



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       they received the Prune(S,G). When the prune-pend timer expires,
       all PEs will remove the downstream (S,G,CE4) states.

       Resulting states at the PEs:

          At PE1:
             JT(AC1,S,G,CE3)        = active
             UpstreamNeighbors(S,G) = { CE3 }
             UpstreamPorts(S,G)     = { PW12 }
             OutgoingPortList(S,G)  = { AC1, AC2, PW12 }

          At PE2:
             JT(PW12,S,G,CE3)       = active
             UpstreamNeighbors(S,G) = { CE3 }
             UpstreamPorts(S,G)     = { AC3 }
             OutgoingPortList(S,G)  = { PW12, AC3 }

          At PE3: no (S,G) state.

   Note that at the end of the assert election, there should be no
   duplicate traffic forwarded downstream and traffic should flow only
   on the desired path.  Also note that there are no unnecessary (S,G)
   states on PE3 after the assert election.

B.2.  PIM Proxy Example with (S,G) / (*,G) interaction

   In the same network, let us assume CE4 is the Upstream Neighbor
   towards the RP for G.

    1. CE1 Sends a Join(S,G) with Upstream Neighbor(S,G) = CE3.
    2. PE1 consumes the Join(S,G). PE1 looks up the neighbor database
       and determines CE3 was learnt on PW12. PE1 sends a Proxy
       Join(S,G) to the resulting UpstreamPorts(G). i.e. it sends the
       proxy Join(S,G) on PW12.
    3. Likewise, PE2 consumes the Join(S,G) and sends a proxy Join(S,G)
       on AC3 with Upstream Neighbor = CE3.

       The resulting states at the PEs is as follows:

          At PE1:
              JT(AC1,S,G,CE3)        = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { PW12 }
              OutgoingPortList(S,G)  = { AC1, PW12 }

          At PE2:
              JT(PW12,S,G,CE3)       = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE3 }



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              UpstreamPorts(S,G)     = { AC3 }
              OutgoingPortList(S,G)  = { PW12, AC3 }

          At PE3:    PE3 did not receive any PIM Join(S,G). So it has
                     no (S,G) state.

    4. The multicast stream (S,G) flows along CE3 -> PE2 -> PE1 -> CE1.
    5. Now let us say CE1 sends a Join(*,G) towards CE4.
    6. PE1 consumes the Join(*,G). PE1 sends a Proxy Join(*,G) to the
       resulting UpstreamPorts(G). Since UpstreamPorts(G) now has both
       PW12 and PW13, the Join(*,G) gets sent on both PW12 and PW13.
       Note that the UpstreamPorts(S,G) and OutgoingPortList(S,G)
       inherit the corresponding (*,G) sets, but not vice versa.
remove "but not vice versa"
COMMENT : > Original "but not vice versa" applies to OutgoingPortList(S,G) only,
I assume, because of the earlier definition:

UpstreamPorts(G): This set is the union of all the
          UpstreamPorts(S,G) and UpstreamPorts(*,G) for a given G


    7. PE2 and PE3 perform a similar function. PE2 received the
       Join(*,G) on a PW and the Upstream Neighbor is also on a PW.
       Hence PE2 only adds UpstreamPorts(*,G) to OutgoingPortList(*,G)
       and not the downstream port PW12.

          At PE1:
              JT(AC1,S,G,CE3)        = active
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { PW12, PW13 }
              OutgoingPortList(S,G)  = { AC1, PW12, PW13 }

              JT(AC1,*,G,CE4)        = JP_HoldTime.
              UpstreamNeighbors(*,G) = { CE4 }
              UpstreamPorts(*,G)     = { PW13 }
              OutgoingPortList(*,G)  = { AC1, PW13 }

              UpstreamPorts(G)       = { PW12, PW13 }

          At PE2:
              JT(PW12,S,G,CE3)       = active
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { AC3, PW23 }
              OutgoingPortList(S,G)  = { PW12, AC3, PW23 }

              JT(PW12,*,G,CE4) = JP_HoldTime
              UpstreamNeighbors(*,G)      = { CE4 }
              UpstreamPorts(G)       = { PW23 }



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              OutgoingPortList(*,G)       = { PW23 }

          At PE3:
              JT(PW13,*,G,CE4) = JP_HoldTime
              UpstreamNeighbors(*,G)      = { CE4 }
              UpstreamPorts(*,G)     = { AC4 }
              OutgoingPortList(*,G)       = { PW13, AC4 }

    8. The above state results in both (S,G) and (*,G) streams to be
       forwarded to AC1. The above state also results in the (S,G)
       stream to be forwarded from CE3 to CE4 resulting in an (S,G)
       assert election. Following the assert election, CE3 becomes the
       (S,G) assert winner. CE4 stops sending (S,G) stream down the
       RPT.
    9. CE1 notices an RPF change due to assert. It sends a
       Prune(S,G,rpt) with Upstream Neighbor = CE4.
   10. PE1 consumes the Prune(S,G,rpt) and forwards the
       Prune(S,G,rpt) to both PW12 and PW13. PE2 consumes the
       Prune(S,G,rpt) and updates its states. PE3 updates its states
       and forwards the Prune(S,G,rpt) on AC4.

          At PE1:
              JT(AC1,S,G,CE3)        = active
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { PW12 }
              OutgoingPortList(S,G)  = { AC1, PW12 }

              JT(AC1,*,G,CE4)        = active.
              UpstreamNeighbors(*,G) = { CE4 }
              UpstreamPorts(*,G)     = { PW13 }
              OutgoingPortList(*,G)  = { AC1, PW13 }

          At PE2:
              JT(PW12,S,G,CE3)       = active
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(*,G)     = { AC3 }
              OutgoingPortList(S,G)  = { PW12, AC3 }

              JT(PW12,*,G,CE4)       = JP_HoldTime
              UpstreamNeighbors(*,G) = { CE4 }
              UpstreamPorts(*,G)     = { PW23 }
              OutgoingPortList(*,G)  = { PW23 }

          At PE3:
              JT(PW13,*,G,CE4)       = JP_HoldTime
              UpstreamNeighbors(*,G) = { CE4 }
              UpstreamPorts(G)       = { AC4 }
              OutgoingPortList(*,G)  = { PW13, AC4 }



Dornon, et al.          Expires January 17, 2013               [Page 38]


Internet-Draft             l2vpn-pim-snooping                  July 2012


   Even in this example, at the end of the (S,G) / (*,G) assert
   election, there should be no duplicate traffic forwarded downstream
   and traffic should flow only to the desired CEs.

   Other more complex scenarios exist.  This draft should addressin PIM-
   SM and the rules specified in this draft should ensure that assert is
   triggered among the CEs in all scenarios.


Authors' Addresses

   Olivier Dornon
   Alcatel-Lucent
   50 Copernicuslaan
   Antwerp, B2018

   Email: olivier.dornon@alcatel-lucent.com


   Jayant Kotalwar
   Alcatel-Lucent
   701 East Middlefield Rd.
   Mountain View, CA 94043

   Email: jayant.kotalwar@alcatel-lucent.com


   Jeffrey Zhang
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, MA 01886

   Email: zzhang@juniper.net


   Venu Hemige
   Alcatel-Lucent
   701 East Middlefield Rd.
   Mountain View, CA 94043

   Email: Venu.hemige@alcatel-lucent.com










Dornon, et al.          Expires January 17, 2013               [Page 39]


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