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Versions: 00 01 02 RFC 3488

INTERNET-DRAFT     Router-port Group Management Protocol           I. Wu
Type: Informational                                            T. Eckert
Expires: December 2002                                     cisco Systems
                                                         Tue, Jun 4 2002

                          Cisco Systems
             Router-port Group Management Protocol (RGMP)

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1] except that the right to
   produce derivative works is not granted.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   This draft documents RGMP, a protocol developed by Cisco Systems
   that is used between multicast routers and switches to restrict
   multicast packet forwarding in switches to those routers where the
   packets may be needed.

   RGMP is designed for backbone switched networks where multiple, high
   speed routers are interconnected.

1. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC2119 [2].

2. Introduction

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   IGMP Snooping is a popular, but not well documented mechanism to
   restrict multicast traffic, in switched networks, to those ports that
   want to receive the multicast traffic.  It dynamically establishes
   and terminates multicast group specific forwarding in switches that
   support this feature.

   The main limitation of IGMP Snooping is that it can only restrict
   multicast traffic onto switch ports were receiving hosts are
   connected directly or indirectly via other switches. IGMP Snooping
   can not restrict multicast traffic to ports where at least one
   multicast router is connected. It must instead flood multicast
   traffic to these ports. Snooping on IGMP messages alone is an
   intrinsic limitation. Through it, a switch can only learn which
   multicast flows are being requested by hosts. A switch can not
   learn through IGMP which traffic flows need to be received by
   router ports to be routed because routers do not report these
   flows via IGMP.

   In situations where multiple multicast routers are connected to a
   switched backbone, IGMP Snooping will not reduce multicast traffic
   load. Nor will it assist in increasing internal bandwidth through
   the use of switches in the network.

   In switched backbone networks or exchange points, where
   predominantly routers are connected with each other, a large amount
   of multicast traffic may lead to unexpected congestion. It also
   leads to more resource consumption in the routers because they must
   discards the unwanted multicast traffic.

   The RGMP protocol described in this document allows to restrict
   multicast traffic to router ports.  To effectively restrict traffic,
   it must be supported both by the switches and the routers in the

   The message format of RGMP resembles that of IGMPv2 so existing
   switches that are capable of IGMP Snooping can easily be enhanced
   with this feature.  Its messages are encapsulated in IP datagrams,
   with an IP protocol number of 2, the same as that of IGMP.  All RGMP
   messages are sent with IP TTL 1, to destination address
   This address has been assigned by IANA to carry messages from
   routers to switches [3].

   RGMP is designed to work in conjunction with multicast routing
   protocols where explicit join/prune to the distribution tree is
   performed.  PIM-SM [4] is an example of such a protocol.

   To keep RGMP simple, efficient and easy to implement, it is designed
   for switches to expect RGMP messages from only one source per

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   port.  For this reason, RGMP only supports a single RGMP enabled
   router to be connected directly to a port of an RGMP enabled switch.
   Such a topology should be customary when connecting routers to
   backbone switches and thus not pose a limitation on the deployment
   of RGMP.

   All RGMP messages have the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |      Type     |   Reserved    |           Checksum            |
   |                         Group Address                         |

   The reserved field in the message MUST be transmitted as zeros and
   ignored on receipt.

2.1 Type

   There are four types of RGMP messages of concern to the router-
   switch interaction.  The type codes are defined to be the highest
   values in an octet to avoid the re-use of already assigned IGMP
   type codes.

      0xFF = Hello
      0xFE = Bye
      0xFD = Join a group
      0xFC = Leave a group

   Unrecognized message types should be silently ignored.


   RGMP and the IANA assignment of address for it predates
   RFC3228 [9]. RGMP defines Type values which in RFC3228 are assigned
   to protocol testing and experimentation. This is not an operational
   issue for RGMP itself because only RGMP packets use the IP
   destination address The Type values defined above
   are thus ONLY valid in conjunction with the RGMP destination address.

2.2. Checksum

   Checksum covers the RGMP message (the entire IP payload).  The
   algorithm and handling of checksum are the same as those for IGMP
   messages as described in RFC2236 [5].

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2.3. Group Address

   In an RGMP Hello or Bye message, the group address field is set to

   In an RGMP Join or Leave message, the group address field holds the
   IP multicast group address of the group being joined or left.

2.4 IP header

   RGMP messages are sent by routers to switches. The source IP address
   of an RGMP packet is the sending interface IP address of the
   originating router.  The destination IP address of an RGMP packet is  Switches supporting RGMP need to listen to packets to
   this group.

3. RGMP Protocol Description

3.1 RGMP Router side Protocol Description

   Backbone switches use RGMP to learn which groups are desired at each
   of their ports.  Multicast routers use RGMP to pass such information
   to the switches.  Only routers send RGMP messages. They ignore
   received RGMP messages.

   A Router enabled for RGMP on an interface periodically [Hello
   Interval] sends an RGMP Hello message to the attached network to
   indicate that it is RGMP enabled.  When RGMP is disabled on a
   routers interface, it will send out an RGMP Bye message on
   that interface, indicating that it again wishes to receive IP
   multicast traffic promiscuously from that interface.

   When RGMP is enabled on an interface, a router sends an RGMP Join
   message out this interface for each group that it wants to receive
   traffic for from the interface.  The router needs to periodically
   [Join Interval] re-send an RGMP Join for a group to indicate its
   continued desire to receive multicast traffic for it.

   Routers supporting RGMP MUST NOT send RGMP Join or Leave messages for
   groups 224.0.0.x (x=0...255), and The latter
   two are known as cisco-rp-announce and cisco-rp-discovery [3].

   When a router no longer needs to receive traffic for a particular
   group, it sends an RGMP Leave message for the group. For robustness,
   the router MAY send more than one such message.

   If IP multicast packets for an undesired group are received at a

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   router from a switch, the router MAY send a RGMP Leave message for
   that group to the switch. These messages are called data-triggered
   RGMP Leave messages and the router SHOULD rate-limit them. The
   router MAY suppress sending a data triggered RGMP Leave message if
   it has a desired group that has the same MAC destination address as
   the undesired group (See RFC1112 [6] for MAC ambiguity). Such
   suppression of data triggered RGMP Leave messages SHOULD be
   configurable if supported.

3.2 RGMP Switch side Protocol Description

   A switch enabled for RGMP on a network consumes RGMP messages
   received from ports of the network and processes them as described
   below. If enabled for RGMP, the switch must NOT forward/flood
   received RGMP messages out to other ports of the network.

   RGMP on a switch operates on a per port basis, establishing per-group
   forwarding state on RGMP enabled ports.  A port reverts into an
   RGMP enabled port upon receipt of an RGMP Hello message on the
   port, and a timer [5 * Hello Interval] is started.  This timer is
   restarted by each RGMP Hello message arriving on the port.  If
   this timer expires or if it is removed by the arrival of an RGMP Bye
   message, then the port reverts to its prior state of multicast
   traffic forwarding.

   Correct deployment of RGMP is one RGMP enabled router directly
   connected to a port on a switch that supports RGMP. The port on the
   switch MAY want to keep track of the IP originator address of the
   RGMP Hello and Bye messages it receives on that port. In the event
   of multiple RGMP IP originating addresses the switch MAY generate
   an alert to notify the administrator. The switch MAY also have a
   configuration option that will allow to disable RGMP in this
   situation and fall back to flooding IP multicast on that port,
   although this is a potentially dangerous option:

   By default, connecting two or more RGMP enabled routers to a switch
   port will cause intermittent black holing of IP multicast traffic
   towards these routers. Black holing occurs when a RGMP Leave is
   received from one router while the other router is still joined.

   This malfunction is not only easily recognized by the actual users
   connected through the routers, but it also adheres to the principle
   of a failure situation to rather cause less traffic than more.
   Reverting to flooding easily maintains the illusion that everything
   is working perfectly, only that the traffic constraining benefits
   of RGMP are not realized and probably congestion happens at a much
   later time than the misconfiguration and can then not easily be
   correlated with the cause anymore.

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   Because routers supporting RGMP are not required to send RGMP Join
   or Leave messages for groups 224.0.0.x (x=0...255), and, RGMP enabled ports always need to receive traffic for
   these groups.  Traffic for other groups is initially not forwarded
   to an RGMP enabled port.

   RGMP Join and Leave messages are accepted if they arrive on an
   RGMP enabled port, otherwise they will be discarded.  Upon acceptance
   of an RGMP Join message, the switch MUST start forwarding traffic for
   the group to the port. Upon acceptance of an RGMP Leave message, the
   switch SHOULD stop forwarding traffic for the group to that port. The
   switches ability to stop forwarding traffic for a group may be
   limited for example because of destination MAC based forwarding in
   the switch, which makes it necessary for the switch to always forward
   traffic for all MAC-ambiguous IP multicast groups (see [5] for

   To stop forwarding of traffic for a group in the event of lost RGMP
   Leave message(s), a switch MAY time out RGMP forwarding state on
   a port for a group [5 * Join Interval] after the last RGMP Join for
   that group has been received on the port.

   By default a switch needs to flood multicast traffic to all ports.
   If a switch supporting RGMP does concurrently also provide for other
   mechanisms to constrain multicast traffic forwarding, then the switch
   must be able to get IP multicast traffic also flooded to a port
   connected directly or indirectly to an IP multicast router.

   Compliance with this specification requires a switch to be able to
   elect a port for flooding through the presence of PIM Hello messages
   [4] arriving from a port and also through a configuration option to
   support routers not supporting PIM. In addition, the switch can of
   course also recognize a port connected to a router by other
   appropriate protocol packets or mechanisms.

   Further mechanisms for IP multicast traffic restriction may also be
   used on RGMP enabled ports. In this case, forwarding for a group on
   the port must be established if either mechanism requires it, and it
   must only be removed if no mechanism requires it anymore.

4.   Operational Notes

4.1. Support for networks with multiple switches

   To be simple to implement on switches and resilient in face of
   potential layer 2 network topology changes, RGMP does not specify how
   to restrict multicast traffic on links connecting switches amongst

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   each other. With just RGMP being used, multicast traffic will thus
   be flooded on inter-switch links within a network if at least
   one router is connected to each of the switches.

   This happens implicitly because the switch will not flood/forward
   received RGMP messages out to the inter-switch link and thus the
   switch on the other end will only recognize the port as a router
   port via the PIM Hello messages flooded by the switches. Manual
   configuration for inter-switch links may be required if non-PIM
   routers are being used, depending on the other capabilities of the

   A switch can of course - if appropriate - send out RGMP messages on
   ports to make it look like an RGMP enabled router to a potential
   switch at the other end of the link. This would allow to constrain IP
   multicast traffic between switches, but this type of "RGMP Spoofing"
   by the switch is outside the scope of this specification.

4.2. Interoperability with RGMP-incapable routers

   Since RGMP messages received at a switch only affect the state
   of their ingress ports, the traffic restriction is applied
   there only.  RGMP-incapable routers will receive multicast
   traffic for all multicast groups.

4.3. RGMP and multicast routing protocols

   One result of the simplicity of RGMP are its restrictions in
   supporting specific routing protocols. The following paragraphs
   list a few known restrictions.

   A router running RGMP on a switched network will not receive traffic
   for a multicast group unless it explicitly requests it via RGMP
   Join messages (besides those group ranges specified to be
   flooded in 3.1).  For this reason, it is not possible to run a
   protocol like PIM Dense-Mode or DVMRP across an RGMP enabled
   network with RGMP enabled routers.

   In Bidir-PIM, a router elected to be the DF must not be enabled
   for RGMP on the network, because it unconditionally needs to forward
   traffic received from the network towards the RP.  If a router is not
   the DF for any group on the network, it can be enabled for RGMP on
   that network.

   In PIM-SM, directly connected sources on the network can not be
   supported if the elected DR is running RGMP, because this DR needs
   to unconditionally receive traffic from directly connected sources
   to trigger the PIM-SM registering process on the DR.  In PIM-SSM,

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   directly connected sources can be supported with RGMP enabled

   Both in PIM-SM and PIM-SSM, upstream routers forwarding traffic
   into the switched network need to send RGMP Joins for the group in
   support of the PIM assert process.

5. List of timers and default values

5.1. Hello Interval

   The Hello Interval is the interval between RGMP Hello messages sent
   by an RGMP-enabled router to an RGMP-enabled switch.  Default:
   60 seconds.

5.2. Join Interval

   The Join Interval is the interval between periodic RGMP Join
   messages sent by an RGMP-enabled router to an RGMP-enabled switch
   for a given group address.  Default: 60 seconds.

6. Security Considerations

   We consider the ramifications of a forged message of each type.

   Hello Message:

      A forged RGMP Hello message from a machine could restrict
      multicast data toward itself.  It has no adverse effect to other

   Bye Message:

      A forged RGMP Bye message from a machine could turn a segment of
      a switched network to be RGMP-disabled.  Even then, this segment
      will get no more multicast data than what it may get without this

   Join Message:

      A forged RGMP Join message from a machine could attract undesired
      multicast packets to the port where it is received.  Even then,
      this port will get no more data than what it may get without this

   Leave Message:

      A forged RGMP Leave message from a machine could restrict

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      multicast data toward itself.  It has no adverse effect to other

   These issues can best be avoided by never connecting multiple systems
   onto a single port in a switched network, or at least by
   never putting a router and hosts onto the same port.

   Misconfiguration as described in section 3.2 - putting more than
   one router on a port - may lead to black holing. In this respect,
   physical integrity of router ports is required to avoid such an

7. References

   1  Bradner, S., "The Internet Standards Process -- Revision 3",
      RFC2026, October 1996.

   2  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", RFC2119, March 1997.

   3  Internet Multicast Addresses,

   4  Estrin, D., et al, "Protocol Independent Multicast-Sparse Mode
      (PIM-SM): Protocol Specification", RFC2362, June 1998

   5  Fenner, W., "Internet Group Management Protocol, Version 2",
      RFC2236, November 1997

   6  Deering, S., "Host Extensions for IP Multicasting", RFC1112,
      August 1989.

   7. ANSI/IEEE Std 802.1D 1998 Edition, "Media Access Control (MAC)
      Bridges", 1998.

   8. Farinacci D., Tweedly D., Speakman T.,
      "Cisco Group Management Protocol (CGMP)", 1996/1997

   9. Fenner, B., "IANA Considerations for IPv4 Internet Group
      Management Protocol (IGMP)", RFC3228, February 2002

8. Acknowledgments

   The authors would like to thank Gorry Fairhurst, Bill Fenner,
   Giovanni Meo, Mike Norton, Pavlin Radoslavov and Alex Zinin for
   their review of the document and their suggestions.

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9. Author's Addresses

   Ishan Wu
   cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134
   Phone: (408) 526-5673
   Email: iwu@cisco.com

   Toerless Eckert
   cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134
   Phone: (408) 853-5856
   Email: eckert@cisco.com

10. Revision History

   A record of changes which will be removed before publication.

   (1) Fixed typographic details as per Gorry Fairhurst.

   (2) Corrected protocol behavior description
      (legacy text that incorrectly made it into -01).
       a) Sending of data triggered RGMP Leave messages and
          their suppression due to MAC ambiguity is optional.
       b) Switches maintaining per group join timers is optional.
       c) Switches may want to recognize misconfiguration (connection
          of more than one router to a port).

   (3) The Intellectual Property Rights appendix (A.) was
       modified to indicate claims.

   (4) An appendix to compare RGMP against GARP/GMRP was added.

   (5) An appendix to discuss possible future extensions and
       a comparison against PIM Snooping was added.

   (6) An acknowledgment section was added.

   (7) A note about RFC3228 and RGMPs use of experimental Type
       fields was added.

   (8) Added security consideration about physical misconfiguration.

Appendix A. Intellectual Property Rights

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   Cisco has submitted a patent claim covering RGMP technology.

Appendix B. Comparison with GARP/GMRP

   This appendix is not part of the RGMP specification but is
   provided for information only.

   GARP/GMRP (defined in IEEE 802.1D [7]) is the ANSI/ISO/IEC/IEEE
   protocol suite to constrain ethernet multicast traffic in bridged
   ethernet networks. As such it is also a possible alternative to
   RGMP for the purpose of constraining multicast traffic towards
   router ports. This appendix will explain the motivation not to
   rely on GARP/GMRP and how GARP/GMRP and RGMP differ.

   The key factor in rolling out GARP/GMRP would have been to
   completely replace IGMP Snooping. This was the design goal of
   GARP/GMRP. For efficient operations, IGMP Snooping requires hardware
   filtering support in the switch (to differentiate between hosts
   membership reports and actual IP multicast traffic). Especially in
   many older switches this support does not exist. Vendors tried to
   find a way around this issue to provide the benefit of constraining
   IP multicast traffic in a switched LAN without having to build more
   expensive switch hardware.  GARP/GMRP is one protocol resulting from
   this. CGMP from Cisco is another one. While CGMP solves the problem
   without requiring changes to the host stack software, GARP/GMRP
   requires support for it by the host stack. This dependency is the
   main reason for GARP/GMRPs missing success in the market.

   Although GARP/GMRP was heavily promoted by individual vendors, it
   has so far not made significant inroads into deployed solutions.
   IGMP Snooping (and CGMP) are the norm for this environment.  In
   result, GARP/GMRP can not necessarily be expected to be supported by
   layer 2 switches. In addition, GARP/GMRP does not address clearly
   the issues RGMP tries to solve.  On one hand, GARP/GMRP provides
   much more functionality and as such complexity as immediately
   required. On the other hand, GARP/GMRP is limited by being a
   standard predominantly for the Ethernet scope.

   Beyond the process and applicability reasons, the main differences
   between GARP/GMRP and RGMP are as follows:

   o GARP/GMRP switches/systems need to send and listen/react to
     GARP/GMRP messages. In RGMP, routers only need to send RGMP
     messages and switches only need to listen to them. This
     protocol approach is meant to simplify implementation,
     operations and troubleshooting.

   o The same switch running RGMP in a backbone network will likely

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     see more states then running on the edge only doing IGMP Snooping,
     making it preferable to keep the amount of per group processing
     and memory requirements in RGMP more in bounds than possible in
     IGMP Snooping and GARP/GMRP: In GARP/GMRP, a (multiple) timer
     based state-machines needs to be maintained on a per ethernet
     group address, in RGMP timer maintenance is completely optional
     and there are only two states per group (joined or not joined).

   o GARP/GMRP is an ethernet level protocol from the IEEE. It supports
     to constrain traffic for ethernet addresses (groups). RGMP does
     constrain traffic for IP multicast groups. Today this is even
     beyond the capabilities of typical switch platforms used as layer2
     switches. Extensions to support further entities are likely easier
     to come by through extensions to RGMP than to GARP/GMRP.

   o RGMP shares the basic packet format with IGMP (version 2)
     and is as such easy to add to router and switch platforms that
     already support IGMP respectively IGMP Snooping. This is
     especially true for switches that in hardware can differentiate
     between IGMP protocol type packets and other IP multicast traffic
     sent to the same (or a MAC ambiguous) group. In addition, due to
     the state simplicity of RGMP it is easy to integrate IGMP
     Snooping and RGMP operations in a switches IP multicast control
     and forwarding plane.

   o GARP/GMRP supports more than one system (host/router) on a
     switch port which is one reason for its complexity. In RGMP,
     this configuration is explicitly not supported: More than one
     router per switched port is not only not a common scenario in
     today's switches layer 2 networks, it is also an undesired
     configuration when unwanted IP multicast traffic is to be
     kept away from routers.

   o GARP/GMRP defines how to constrain multicast traffic between
     switches, another reason for its complexity. RGMP does not
     explicitly support this as part of the protocol because of
     the following reasons:

       o It is not necessary to include this function as part of the
         RGMP protocol description because switch implementations
         can transparently decide to support this function (see
         4.1 about this "RGMP Spoofing").

       o Important deployments through which large amounts of IP
         multicast are moved today are typically single switch.
         MIX - Multicast Internet eXchange points.

       o Avoiding congestion on inter-switch links in general is more

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         complex than simply constraining ip multicast traffic to paths
         where it is needed. With or without IP multicast, the aggregate
         bandwidth needed between switches can easily be the aggregate
         required bandwidth to routers on either sides. For this
         reason, inter-switch bandwidth is most often appropriately
         over provisioned. In addition, the likelihood for receiving
         routers to be only on the sources side of an inter-switch
         link is in general deployments rather low. The cases where
         traffic constrainment on inter-switch links is required and
         helpful is thus limited and can in most cases be avoided or
         worked around. Moving the network to a layer 3 routed network
         is often the best solution, supporting RGMP-Spoofing (see
         section 4.1) is another one.

Appendix C. Possible future extensions / comparison to PIM Snooping

   This appendix is not part of the RGMP specification but is
   provided for information only.

   This appendix presents a discussion of possible extensions to
   RGMP. Included are points on why the extensions are not included
   and in addition a motivation for RGMP in comparison to (PIM)

   o Support for multiple switches

     As discussed in "RGMP Spoofing", chapter 4.1 and GARP/GMRP
     comparison in Appendix B.

   o Support for SSM

     While RGMP works with PIM-SSM, it does not have explicit messages
     for the router to selectively join to (S,G) channels individually.
     Instead the router must RGMP join to all (Si,G) channels by
     joining to G. Extending RGMP to include (S,G) Join/Leaves is
     feasible. However, currently the majority of switches do not
     support actual traffic constraining on a per channel basis. In
     addition, the likelihood for actual channel collision (two SSM
     channels using the same group) will only become an issue when SSM
     is fully deployed.

   o Support for IPv6

     RGMP could easily be extended to support IPv6 by mapping
     the RGMP packet format into the MLD/IPv6 packet format. This
     was not done for this specification because most switches today
     do not even support MLD snooping.

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   o Support for multiple routers per port

     As discussed in Appendix B. This is probably one extension
     that should be avoided. Multiple RGMP router per port
     are inappropriate for efficient multicast traffic constrainment.

   o Support for non-join based protocols / protocol elements

     For protocols like PIM dense-mode, DVMRP or Bidir-PIM DF routers,
     additional RGMP messages may be added to allow routers to indicate
     that certain group (ranges) traffic need to be flooded from
     (dense-mode) or to (Bidir-PIM) them.

   o Support for multi-policy switching

     In Multicast Exchange Points (MIXes) environments situations exist
     where different downstream routers for policy reasons need to
     receive the same traffic flow from different upstream routers.

     This problem could be solved by actually providing an upstream
     neighbor field in RGMP Join/Leave messages. The RGMP switch would
     then forward traffic from one upstream router only to those
     downstream routers who want to have the traffic from exactly this
     upstream router.  This extension would best go in hand with
     changes to the layer 3 routing protocol run between the routers.

   As previously mentioned, RGMP was designed to be easy to
   implement and to support simple layer2 switches. Implementations
   could also be applied to switches beyond layer 2. If all the above
   possible future extensions were to be supported by an evolution of
   RGMP, it would be questionable whether such a protocol could be any
   less complex than actually snooping into the layer3 IP routing
   protocol run between routers in a switched LAN.

   From the perspective of protocol architecture it is certainly
   more appropriate to have a separate protocol like RGMP or GARP/GMRP
   for this purpose. Then again, the more complex the requirements are,
   the more duplication of effort is involved and snooping seems to
   become a more attractive option.

   Even though there exists one predominant routing Protocol, PIM, in
   IP multicast, routing with PIM in itself is extremely complex for a
   switch to snoop into. PIM has two main versions, different modes
   - sparse, dense, bidir, ssm, join / prune / graft messages
   (depending on the mode of the group), various PIM Hello options,
   different versions of asserts, two dynamic mode announcement
   protocols (BSR, AutoRP), and finally supports both IPv4 and IPv6.

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INTERNET-DRAFT     Router-port Group Management Protocol      Jun 4 2002

   A switch snooping into PIM is very likely to implement just a subset
   of this feature set, making it very hard for the user to determine
   what level of actual traffic constrainment is achieved unless a
   clear specification exists for the implementation (or better the
   method per se.). In addition, there is always the danger that such
   a snooping implementation may break newer features of the routing
   protocol that it was not designed to handle (likely because they
   could not have been predicted). Exactly this, for example, happens
   with switches using IGMP (v2) snooping implementations are being
   subjected to IGMP version 3 messages - they break IGMPv3.

   In summary, with PIM still evolving, the approach taken by RGMP
   is the safest one for the immediate problems at hand, and
   extensions like those listed should be considered in time for
   actual demand. (PIM) snooping is a valid alternative once the
   total amount of features that need to be supported makes it
   an equally attractive solution (with respect to complexity) to a
   dedicated protocol and if its functions are well defined to allow
   predicting its effects - but always at the price of possible
   incompatibilities with upcoming PIM protocol extensions unless
   support for layer 2 switches is explicitly considered in moving
   PIM protocols forward.

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