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Network Working Group I. Wu Request for Comments: 3488 T. Eckert Category: Informational Cisco Systems

                                                         February 2003
                           Cisco Systems
            Router-port Group Management Protocol (RGMP)

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.


 This document describes the Router-port Group Management Protocol
 (RGMP).  This protocol was developed by Cisco Systems and is used
 between multicast routers and switches to restrict multicast packet
 forwarding in switches to those routers where the packets may be
 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",
 document are to be interpreted as described in BCP 14, RFC 2119 [2].

2. Introduction

 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.

Wu & Eckert Informational [Page 1] RFC 3488 Cisco Systems RGMP February 2003

 The main limitation of IGMP Snooping is that it can only restrict
 multicast traffic onto switch ports where 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 discard the
 unwanted multicast traffic.
 The RGMP protocol described in this document restricts multicast
 traffic to router ports.  To effectively restrict traffic, it must be
 supported by both the switches and the routers in the network.
 The RGMP message format resembles the IGMPv2 message format so that
 existing switches, capable of IGMP Snooping, can easily be enhanced
 with this feature.  Its messages are encapsulated in IPv4 datagrams,
 with a protocol number of 2, the same as that of IGMP.  All RGMP
 messages are sent with 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.
 The RGMP protocol specifies operations only for IP version 4
 multicast routing.  IP version 6 is not considered.
 To keep RGMP simple, efficient and easy to implement, it is designed
 for switches to expect RGMP messages from only one source per 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.

Wu & Eckert Informational [Page 2] RFC 3488 Cisco Systems RGMP February 2003

 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
 RFC 3228 [9].  RGMP defines Type values which in RFC 3228 are
 assigned to protocol testing and experimentation.  This is not an
 operational issue for RGMP itself because only RGMP packets use the
 IPv4 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 IPv4 payload).  The
 algorithm and handling of checksum are the same as those for IGMP
 messages as described in RFC 3376 [5].

Wu & Eckert Informational [Page 3] RFC 3488 Cisco Systems RGMP February 2003

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
 IPv4 multicast group address of the group being joined or left.

2.4 IPv4 header

 RGMP messages are sent by routers to switches. The source IPv4
 address of an RGMP packet is the sending interface IPv4 address of
 the originating router.  The destination IPv4 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 IPv4 multicast traffic
 promiscuously from that interface.
 When an interface is RGMP enabled, a router sends an RGMP Join
 message out through this interface to 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.
 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.

Wu & Eckert Informational [Page 4] RFC 3488 Cisco Systems RGMP February 2003

 If IPv4 multicast packets for an undesired group are received at a
 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 RFC 1112 [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 IPv4 originator address of the
 RGMP Hello and Bye messages it receives on that port.  In the event
 it receives multiple IPv4 originating addresses in RGMP messages on
 one port, the switch MAY generate an alert to notify the
 administrator.  The switch MAY also have a configuration option that
 will allow for the operator to disable RGMP and have the switch fall
 back to flooding IPv4 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 IPv4 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
 that a failure situation causes less traffic than more.  Reverting to
 flooding easily maintains the illusion that everything is working
 perfectly.  The exception is that the traffic constraining benefits

Wu & Eckert Informational [Page 5] RFC 3488 Cisco Systems RGMP February 2003

 of RGMP are not realized.  This suggests that congestion happens at a
 much later time than the misconfiguration and can then not easily be
 correlated with the cause anymore.
 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 switch's ability to stop forwarding traffic for a group may be
 limited, for example, because of destination MAC based forwarding in
 the switch.  Therefore, it is necessary for the switch to always
 forward traffic for all MAC-ambiguous IPv4 multicast groups (see [6]
 for MAC-ambiguity).
 To stop forwarding of traffic to 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.
 Without any layer 2 IPv4 multicast filtering methods running, a
 switch needs to flood multicast traffic to all ports.  If a switch
 does actually run one or more mechanisms beside RGMP to filter IPv4
 multicast traffic, such as IGMP snooping [10], then by default it
 will not flood IPv4 multicast traffic to all ports anymore.  Instead,
 the switch will try to determine which ports still needs to receive
 all IPv4 multicast traffic by default, and which ports do not.
 Compliance with this specification requires that a switch MUST be
 able to elect a port for flooding through the presence of PIM Hello
 messages [4] arriving from the port and also through a manual
 configuration option.  In addition, the switch SHOULD recognize a
 port connected to a router by other appropriate protocol packets or
 dedicated IPv4 multicast router discovery mechanisms such as MRDISC
 [11].  The manual configuration is required to support routers not
 supporting PIM or other methods recognized by the switch.
 Further mechanisms for IPv4 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.

Wu & Eckert Informational [Page 6] RFC 3488 Cisco Systems RGMP February 2003

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
 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
 If appropriate, a switch can 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 constrain IPv4 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

Wu & Eckert Informational [Page 7] RFC 3488 Cisco Systems RGMP February 2003

 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,
 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

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

 The RGMP protocol assumes that physical port security can be
 guaranteed for switch ports from which RGMP messages are accepted.
 Physical port security for RGMP means that physical measures will
 ensure that such ports are dedicatedly connected to one system which
 acts as an RGMP capable router.  This is also the recommended
 configuration to best leverage the benefits of the RGMP protocol
 (e.g., avoiding unwanted third-party IPv4 multicast traffic arriving
 on said ports).
 RGMP specific DoS attacks arise from forged RGMP messages.  If more
 than one system is connected to a port of the RGMP switch, then one
 system may forge RGMP messages and affect the operations of the other
 system(s) on the same port.  This is a potential security risk.

Wu & Eckert Informational [Page 8] RFC 3488 Cisco Systems RGMP February 2003

 When physical security ensures that only one system is connected to a
 RGMP capable port on a switch, then forged messages from this system
 itself can take effect.  Such forged messages can always be avoided
 by system local measures.
 We consider the ramifications of a forged message of each type:
 Hello Message:
    A forged RGMP Hello message can restrict multicast data towards a
    non-RGMP enabled router on the same port.  This effectively
    introduces a blackholing DoS attack.
 Leave Message:
    A forged RGMP Leave message can restrict IPv4 multicast traffic
    for individual groups toward the port.  The effect is a possible
    blackholing DoS attack similar to an RGMP Hello Message except
    that it does not affect all IPv4 multicast traffic but only that
    of the groups indicated in the forged messages.  It will also only
    affect a port if there officially is only one RGMP enabled router
    connected to it (i.e., if the port is RGMP enabled).
 Bye Message:
    A forged RGMP Bye message can turn the port into being
    RGMP-disabled.  This could, indirectly, cause a DoS attack based
    on the port getting overloaded with IPv4 multicast traffic if the
    network bandwidth of the port was provisioned with the expectation
    that RGMP will suppress unwanted IPv4 multicast messages.
    This type of DoS attack simply re-establishes a port behavior as
    if RGMP was not configured and invalidates the benefit of RGMP.
    This, however, does not introduce an issue that would not have
    been there without RGMP in the first place.
 Join Message:
    A forged RGMP Join message could attract undesired multicast
    packets to the port where it is received from.  The effect is
    similar to an RGMP Bye Message except that it does not affect all
    IPv4 multicast traffic only the groups indicated in the forged
    messages. The message will affect a port only if there officially
    is only one RGMP enabled router connected to it (i.e., if the port
    is RGMP enabled).

Wu & Eckert Informational [Page 9] RFC 3488 Cisco Systems RGMP February 2003

7. Normative References

 [1]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.
 [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [4]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,
      Handley, M., Jacobson, V., Liu, C., Sharma, P. and L. Wei,
      "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
      Specification", RFC 2362, June 1998.
 [5]  Cain, B., Deering, S., Kouvelas, I., Fenner, W. and A.
      Thyagarajan, "Internet Group Management Protocol, Version 3",
      RFC 3376, October 2002.
 [6]  Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
      1112, August 1989.
 [7]  ANSI/IEEE Std 802.1D 1998 Edition, "Media Access Control (MAC)
      Bridges", 1998.

8. Informative References

 [3]  Internet Multicast Addresses,
 [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)", RFC 3228, February 2002.
 [10] Christensen, M. and F. Solensky, "IGMP and MLD snooping
      switches", Work In Progress.
 [11] Biswas, S., Cain, B. and B. Haberman, "IGMP Multicast Router
      Discovery", Work In Progress.

9. 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.

Wu & Eckert Informational [Page 10] RFC 3488 Cisco Systems RGMP February 2003

Appendix A. Intellectual Property Rights

 The IETF has been notified of intellectual property rights claimed in
 regard to some or all of the specification contained in this
 document.  For more information consult the online list of claimed

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 IPv4 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 IPv4
 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.
 Up to date GARP/GMRP 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.

Wu & Eckert Informational [Page 11] RFC 3488 Cisco Systems RGMP February 2003

 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
 o  The same switch running RGMP in a backbone network will likely 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 IPv4 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
 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 and IGMP Snooping respectively.  This is especially
    true for switches that in hardware can differentiate between IGMP
    protocol type packets and other IPv4 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 the IPv4 multicast control and forwarding plane of a
 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 IPv4 multicast traffic is to be kept away from

Wu & Eckert Informational [Page 12] RFC 3488 Cisco Systems RGMP February 2003

 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 IPv4
       multicast are moved today are typically single switch
       MIX - Multicast Internet eXchange points.
    o  Avoiding congestion on inter-switch links in general is more
       complex than simply constraining IPv4 multicast traffic to
       paths where it is needed.  With or without IPv4 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) snooping.
 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

Wu & Eckert Informational [Page 13] RFC 3488 Cisco Systems RGMP February 2003

    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.
 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 IPv4 routing protocol run between
 routers in a switched LAN.

Wu & Eckert Informational [Page 14] RFC 3488 Cisco Systems RGMP February 2003

 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
 IPv4 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.
 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).  For example, this can happen with switches using
 IGMP (v2) snooping implementations that 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.

Wu & Eckert Informational [Page 15] RFC 3488 Cisco Systems RGMP February 2003

Authors' Addresses

 Ishan Wu
 cisco Systems
 170 West Tasman Drive
 San Jose, CA 95134
 Phone: (408) 526-5673
 Toerless Eckert
 cisco Systems
 170 West Tasman Drive
 San Jose, CA 95134
 Phone: (408) 853-5856

Wu & Eckert Informational [Page 16] RFC 3488 Cisco Systems RGMP February 2003

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Wu & Eckert Informational [Page 17]

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