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Versions: 00 01                                                         
IDMR Working Group                                        M. Christensen
Internet Draft                                                   Vitesse
June 2001                                                    F. Solensky
Expiration Date: December 2001                           Gotham Networks

                     IGMP and MLD snooping switches

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [RFC2026].

   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-Drafts.

   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 memo describes the interoperability problems and issues that can
   arise when a mixed deployment of IGMPv3 and IGMPv2 capable hosts and
   routers are interconnected by a switch with IGMP snooping
   capabilities. The memo also covers MLDv2 for IPv6.  It is intended as
   an accompanying document to the IGMPv3 and MLDv2 specifications.

   The memo contains a brief IGMP walk through followed by a description
   of the IGMP snooping functionality.  Specific examples are given
   which are all based on Ethernet as the link layer protocol. MLDv2 for
   IPv6 is discussed.  Finally recommendations are given for the
   behavior of IGMP snooping switches.

   The purpose of this document is twofold:

     - We want to summarize IGMP snooping induced problems and best cur-
       rent solutions. We hope that a description of IGMP snooping will

Christensen, Solensky                                           [Page 1]

RFC DRAFT                                                      June 2001

       be of aid to the IETF when standardizing new protocols and behav-
       iors within this scope.

     - We also hope to bring this work to the attention of switch ven-
       dors, typically active within the IEEE community but perhaps not
       within IETF, in order to  minimize protocol interoperability
       problems in the future.

1.  Introduction

   In recent years, a number of commercial vendors have introduced prod-
   ucts described as "IGMP snooping switches" to the market.  These
   devices do not adhere to the conceptual model that provides the
   strict separation of functionality between different communications
   layers in the ISO model and instead utilizes information in the upper
   level protocol headers as factors to be considered in the processing
   at the lower levels.  This is analogous to the manner in which a
   router can act as a firewall by looking into the transport protocol's
   header before allowing a packet to be forwarded to its destination

   In the case of multicast traffic, an IGMP snooping switch provides
   the benefit of conserving bandwidth on those segments of the network
   where no node has expressed interest in receiving packets addressed
   to the group address.

   The discussions in this document are based on IGMP which applies to
   IPv4 only. For IPv6 we must use MLD in stead. Because MLD is based on
   IGMP with only a few differences these discussions also apply to

2.  IGMP snooping overview

   For a full description of IGMP we refer to [IGMPv3], however, IGMP
   operation is summarized in the following:

     * Hosts send IGMP Membership Report messages to inform neighboring
       routers that they wish to join a specific IP multicast group.

     * IGMPv3 Membership Reports may be qualified with a list of allowed
       or forbidden source addresses.

     * Routers periodically send IGMP Query messages to hosts in order
       to maintain group membership state information.  These queries
       can be either general or group specific queries.

Christensen, Solensky                                           [Page 2]

RFC DRAFT                                                      June 2001

     * Hosts respond to queries with Membership Reports.

     * Hosts running either IGMPv2 or IGMPv3 may also send a Leave Group
       message to routers to withdraw from the group.

   A traditional Ethernet network may be separated into different net-
   work segments to prevent placing too many devices onto the same
   shared media.  These segments are connected by bridges and switches.
   When a packet with a broadcast or multicast destination address is
   received, the switch will forward a copy into each of the remaining
   network segments in accordance with the IEEE MAC bridge standard
   [BRIDGE].  Eventually, the packet is made accessible to all nodes
   connected to the network.

   This approach works well for broadcast packets that are intended to
   be seen or processed by all connected nodes.  In the case of multi-
   cast packets, however, this approach could lead to less efficient use
   of network bandwidth, particularly when the packet is intended for
   only a small number of nodes.  Packets will be flooded into network
   segments where no node has any interest in receiving the packet.
   While nodes will rarely incur any processing overhead to filter pack-
   ets addressed to unrequested group addresses, they are unable to
   transmit new packets onto the shared media for the period of time
   that the multicast packet is flooded.

   The problem of wasting bandwidth is even worse when the LAN segment
   is not shared, for example in Full Duplex links.  Full Duplex is
   standard today for most switches operating at 1Gbps, and it will be
   standard for 10Gbps ethernet too. In this case the wasted bandwidth
   is proportional to the number of attached nodes.

   Allowing switches to snoop IGMP packets is a creative effort to solve
   this problem.  The switch uses the information in the IGMP packets as
   they are being forwarded throughout the network to determine which
   segments should receive packets directed to the group address.

   IGMP snooping is being implemented slightly different by different
   switch vendors. We will not address specific implementations here as
   documentation is not widely available. For details of one implementa-
   tion we refer to [CISCO].

   In the following we will describe problems in relation to IGMP snoop-
   ing with the following constraints, which we believe are the most
   common cases.

     1. Group membership is based on multicast MAC addresses only.

Christensen, Solensky                                           [Page 3]

RFC DRAFT                                                      June 2001

     2. Forwarding is based on a 'list' of member ports for each sup-
        ported multicast group.

     3. The switch is equipped with a CPU for maintaining group member-
        ship information.

   Constraint 3 above is not a strict requirement as IGMP snooping could
   be accomplished entirely in hardware. However, when sending IGMP
   datagrams all is done to ensure that the packets are not routed. For
   example the TTL is set to 1 and the IP header contains the router
   alert option.  This is a hint to developers that there is probably a
   need to send this packet to the CPU.

   IGMP snooping switches build forwarding lists by listening for (and
   in some cases intercepting) IGMP messages.  Although the software
   processing the IGMP messages may maintain state information based on
   the full IP group addresses, the forwarding tables are typically
   mapped to link layer addresses.  An example of such a forwarding
   table is shown in Figure 1.

                    Multicast MAC address  | Member ports
                    01-00-5e-00-00-01      | 2, 7
                    01-00-5e-01-02-03      | 1, 2, 3, 7
                    01-00-5e-23-e2-05      | 1, 4
                                  Figure 1.

   Because only the least significant 23 bits of the IP address are
   mapped to Ethernet addresses [RFC1112], there is a loss of informa-
   tion when forwarding solely on the destination MAC address.  This
   means that for example and and similar IP
   multicast addresses all map to MAC address 01-00-5e-00-00-7b (for
   Ethernet).  As a consequence, IGMP snooping switches may collapse IP
   multicast group memberships into a single Ethernet multicast member-
   ship group.

   Finally, it should be mentioned that in addition to building and
   maintaining lists of multicast group memberships the snooping switch
   should also maintain a list of multicast routers. When forwarding
   multicast packets they should be forwarded on ports which have joined
   using IGMP but also on ports on which multicast routers are attached.
   The reason for this is that in IGMP there is only one active querier.
   This means that all other routers on the network are suppressed and
   thus not detectable by the switch.

Christensen, Solensky                                           [Page 4]

RFC DRAFT                                                      June 2001

2.1.  Problems in older networks

   The drawback of using IGMP snooping switches to make the flooding of
   multicast traffic more efficient is that the underlying link layer
   topology is required to remain very stable.  This is especially true
   in IGMP versions 1 and 2 where group members do not transmit Member-
   ship Report messages after having overheard a report from another
   group member.

   This problem can be demonstrated with an example. In the topology
   illustrated in figure 2, a topology loop exists between four IGMP
   snooping switches labeled A, B, C and D.

     - The spanning tree algorithm would detect this loop and disable
       one of the links; for example, the link connecting ports B3 and

     - Host H1 transmits a group Membership Report which will be flooded
       throughout the network.

     - When switch A hears the report, it determines that packets
       addressed to the group should be forwarded to port A3.

     - Router R hears the Join message and starts forwarding packets
       with the multicast destination address into the network.  Host H1
       is now part of the group.

     - The link between D2 and C2 is broken.  The spanning tree algo-
       rithm reactivates the blocked link B3-C1.

     - If switch A relies solely on the exchange of IGMP messages to
       alter its forwarding behavior, host H1 will be unable to receive
       packets forwarded to the group address until router R sends out
       another Membership Query.

One possible approach to work around this limitation would be for the
switch to keep track of which nodes belong to the group, altering the
forwarding tables whenever a member becomes visible through a different
port.  When switch A sees that host H1 has moved from port A3 to A2, the
group membership table would be updated. This does not work, however,
when more than one node joins the same group address when at least one
of them has not yet been upgraded to IGMPv3: if hosts H1 and H2 were to
join the group at approximately the same time, they would both start off
random timers for the transmission of their first Membership Reports.
If host H2 selects a longer interval than H1, it will hear H1's report
message and cancel the one it was about to send.  Switch A, therefore,

Christensen, Solensky                                           [Page 5]

RFC DRAFT                                                      June 2001

                                +------+ B2
                             B1 |Snoop |----- - - -          +------+
                           -----|Switch|                     | Host |
                          /     |  B   |----- /              |  H1  |
          +--------+ A2  /      +------+ B3  X  C1 +------+  +--+---+
       A1 | Snoop  |-----                   / -----|Snoop |     |
   --+----| Switch |                               |Switch|-----+----
     |    |   A    |-----                     -----|  C   | C3
   +-+-+  +--------+ A3  \      +------+ D2  /  C2 +------+
   | R |                  \  D1 |Snoop |-----
   ++-++                   -----|Switch|
    | |                         |  D   |---------+------ - - -
                                +------+ D3      |
                                              | Host |
                                              |  H2  |
                               Figure 2
never learns that node H2 has joined the group.  When the switch learns
that H1 is now accessible through port A2, it has no way of knowing that
it should continue forwarding group packets to port A3 as well.

Two recommendations can be made based on the above discussion:

     - The switch should play an active role when detecting a topology
       change; The spanning tree root bridge (which is also a snooping
       switch) should initiate the transmission of a IGMP General Query,
       for example through signalling the CPU. This will help to reduce
       the join latency otherwise introduced.

     - IGMP Membership Reports should not be flooded because this will
       lead to Join suppression.

2.2.  IGMPv2 snooping and 224.0.0.X

   Special attention should be brought to the IP address range from through which is reserved for routing protocols
   and other low-level topology discovery or maintenance protocols
   [IANA].  Examples of reserved multicast addresses are:

   Multicast routers are discouraged from routing packets when a desti-
   nation address falls within this range, regardless of the TTL value.
   The router will be the originator or consumer of these messages so it
   has less of a motivation to maintain forwarding path information for
   these addresses.  As a result, it becomes less critical for the

Christensen, Solensky                                           [Page 6]

RFC DRAFT                                                      June 2001

           All Routers on this Subnet
           (M)OSPF routers
           (M)OSPF DRs
           RIP2 Routers
          PIM Routers
          IGMPv3 Membership Reports
   router to send out periodic Query messages for these groups.  If the
   router chooses not to, the group would be unable to recover from
   topology changes as described above.  Note that the only difference
   between the 'all hosts' address ( and the remainder of this
   range is that the router has no discretion in the former case: it
   MUST NOT send Queries.

   To avoid this situation, IGMP snooping switches should be less con-
   servative when forwarding packets to these addresses and flood them
   to all ports.

   As an example of this, it is reported in [MSOFT] that a number of
   switches can be misconfigured to perform IGMP snooping and forwarding
   for all IP multicast groups:

   Figure 3 illustrates the scenario where two routers R1 and R2 are
   communicating using for example  The routers never send
   IGMP Joins for this address. The switch floods the (unknown) multi-
   cast traffic on all ports.

   Now the server SVR is started and it sends an IGMP Join for, which is snooped by the switch.  The switch then generates
   a Membership Query on all ports to determine which ports have devices
   attached that also belong to this group.

   The routers R1 and R2 do not respond and the switch builds a forward-
   ing port list with only SVR in it.  Now R1 and R2 are not able to
   communicate using this address.

                        +----+     +----------+
                      --| R1 |-----|          |
                        +----+     | Snooping |    +-----+
                                   |          |----| SVR |
                        +----+     |  switch  |    +-----+
                      --| R2 |-----|          |
                        +----+     +----------+

                                    Figure 3.

   There are two possible fixes to this problem: One is to require that
   all routers (also being hosts) which use IP multicast respond to IGMP

Christensen, Solensky                                           [Page 7]

RFC DRAFT                                                      June 2001

   queries in the range 224.0.0.X. This seems unnecessary as discussed
   above because of the inherent link local scope of these messages.

   Another solution to this problem, which is also discussed above,  is
   that the switch is configured to forward all packets for a range of
   IP multicast addresses to all ports (flooding).

   It is suggested that all multicast packets in the range
   through are forwarded on all ports. This of course
   requires an examination of the network layer header.  Note that these
   are IP adress ranges and that mapping these to MAC address range
   01-00-5e-00-00-X is subject to problems discussed in the previous

2.3.  IGMPv3 and IGMPv2 coexistence

   IGMPv3 and IGMPv2 are designed to interoperate with older versions of
   IGMP. Both hosts and routers are capable of falling back to an ear-
   lier version when receiving older IGMP messages, thus enabling a
   mixed deployment and migration to new versions. While this works fine
   in a network of hosts and routers an IGMP snooping switch introduces

   In figure 4 where hosts H1 and H2 are connected to an IGMP snooping
   switch on ports P1 and P2 respectively, consider the following
   sequence of communication:

     - Router R sends an IGMPv3 Query

     - Host H1 sends an IGMPv2 Report since it has only implemented v2.
       R notices this and switches to IGMPv2 mode. The report is not
       received by H2 because of the snooping functionality.

     - Switch S puts H1's port P1 in the forwarding table.

     - Host H2 sends an IGMPv3 Report in response to R's Query.

     - Switch S fails to add H2's port P2 to the forwarding table
       because it doesn't support IGMPv3.

     - H2 does not receive any traffic before R sends its next Query
       which will put H2 in IGMPv2 mode.

This introduces a Join latency for host H2, which apparently cannot be
avoided. The latency is potentially of the order of minutes. It is

Christensen, Solensky                                           [Page 8]

RFC DRAFT                                                      June 2001

possible however to reduce this latency by tuning the Query Interval
which defaults to 125 seconds.

When operating in a mixed deployment mode it is suggested that initially
the Query Interval is set to "a low value" until the compatibility modes
have stabilized both host and routers on the same IGMP version. After
stabilization the Query Interval could be increased to its original

2.4.  Source Specific Joins

   Even for IGMPv3 snooping capable switches there can be limitations
   caused by link layer based forwarding.  This is illustrated in figure

   Assume that host H1 sends a Join(S1, G) to R and that host H2 sends a
   Join(S2, G) to R.

   The switch adds both hosts to the forwarding list for group G.

   Frames originating from sources S1 and S2 for the same multicast
   address G are routed via R. These are sent from R with the router's
   MAC address as source.

   The switch is unable to distinguish the two different types of flow
   and forwards both flows to both hosts. This effectively disables the
   Join source functionality in this network configuration.

                 +----+   P1+----------+
                 | H1 |-----|          |
                 +----+     | Snooping |    +---+   (S1, G)
                            |          |----| R |---  and
                 +----+     |  switch  |    +---+   (S2, G)
                 | H2 |-----|          |
                 +----+   P2+----------+

                                         Figure 4.

   This is a problem caused by layer 2 based forwarding of a layer 3
   flow in conjunction with the difference between the link layer and
   the network layer information.

   The example above means that host implementations cannot rely on the
   router to perform all source address filtering. Therefore they must
   still filter out packets that do not match the source address

Christensen, Solensky                                           [Page 9]

RFC DRAFT                                                      June 2001

   criterion specified in the Join messages.  While this might be seen
   as an inconvience, this is no different than the case where the
   router is directly connected to both hosts on a shared LAN and no
   snooping switch is present.

   An complete solution would be for the switch to further qualify the
   search process by including the source IP address when determining
   which ports should forward the packet.

   Similar problems occur with the attempt to exclude sources.

3.  Snooping Requirements

   Note that in the following we provide suggestions for good/best prac-
   tices when designing IGMP snooping devices. Keywords as MUST, SHOULD,
   MUST NOT etc. are suggestions only.

   1) All IGMP packets (IP packets with IP-PROTO = 2) SHOULD be redi-
   rected to the CPU for IGMP snooping processing and table management.
   This allows for the most flexible IGMP snooping solution.

   2) The switch that provides support for IGMP snooping MUST forward
   all unrecognized IGMP messages and MUST NOT attempt to make use of
   any information beyond the end of the network layer header.  In par-
   ticular, messages where any reserved fields are non-zero MUST NOT be
   subject to "normal" snooping since this could indicate an incompati-
   ble change to the message format.

   3) Packets with a destination IP address in the 224.0.0.X range which
   are *not* IGMP SHOULD be forwarded on all ports.

   4) Packets with a destination IP address outside 224.0.0.X which are
   *not* IGMP SHOULD be forwarded according to port membership tables
   and MUST also be forwarded on router ports.

   5) If a switch receives a *non* IGMP multicast packet without having
   first processed Membership Reports for the group address, it MUST
   forward the packet on all ports.  In other words, the switch must
   allow for the possibility that connected hosts and routers have been
   upgraded to support future versions or extensions of IGMP that the
   switch does not yet recognize. A switch MAY have a configuration
   option that suppresses this operation, but default behavior MUST be
   to allow flooding of unregistered packets.

Christensen, Solensky                                          [Page 10]

RFC DRAFT                                                      June 2001

   6) A snooping switch SHOULD forward IGMP Membership Reports on router
   "ports" only.

   7) The switch supporting IGMP snooping MUST maintain a list of multi-
   cast routers. This list SHOULD be built using IGMP Multicast Router
   Discovery [MRDISC] which is currently going through IETF Last Call.
   IGMP snooping switches MAY build this list based on the arrival port
   for packets destined to 224.0.0.X, when

     - The packets are IGMP Queries or

     - The packets are *not* IGMP or

     - The ports are configured (by management) as having multicast
       routers attached

8) IGMP snooping switches MAY maintain forwarding tables based on either
MAC addresses or IP addresses. If a switch supports both types of for-
warding tables then the default behavior SHOULD be to use IP addresses.

9) Switches which rely on information in the IP header MAY verify that
the IP header checksum is correct.

10) IGMP snooping switches SHOULD inform the CPU (or hardware) when a
link layer topology change has been detected.  Following a topology
change the switch SHOULD initiate the transmission of a General Query on
all ports in order to reduce Join latency.

4.  IPv6 Considerations

   In order to avoid confusion, the previous discussions have been based
   on IGMPv3 functionality which only applies to IPv4 multicast.  In the
   case of IPv6 most of the above discussions are still valid with a few
   exceptions which we will describe here.

   In IPv6 the protocol for multicast group maintenance is called Multi-
   cast Listener Discovery (MLDv2).  IPv6 is not widely deployed today
   and neither is IPv6 multicast.  However, it is anticipated that at
   some time IPv6 switches capable of MLD snooping will appear.

   The three main differences between IGMPv3 and MLDv2 are

     - MLDv2 uses ICMPv6 message types instead of IGMP message types.

     - The ethernet encapsulation is a mapping of 32bits of the 128bit
       DIP addresses into 48bit DMAC addresses [IPENCAPS].

Christensen, Solensky                                          [Page 11]

RFC DRAFT                                                      June 2001

     - Multicast router discovery is done using Neighbor Discovery Pro-
       tocol (NDP) for IPv6. NDP uses ICMPv6 message types.

A minor difference which applies to the requirements section is that in
IPv6 there is no checksum in the IP header. This is the reason that the
checksum validation requirement is listed as a MAY.

The fact that MLDv2 is using ICMPv6 adds new requirements to a snooping
switch because ICMPv6 has multiple uses aside from MLD. This means that
it is no longer sufficient to detect that the next-header field of the
IP header is ICMPv6 in order to redirect packets to the CPU.  If this
was the case the CPU queue assigned for MLD would potentially be filled
with non-MLD related packets. Furthermore ICMPv6 packets destined for
other hosts would not reach their destination.

A solution is either to require that the snooping switch looks further
into the packets or to be able to detect a multicast DMAC address in
conjunction with ICMPv6.

The first solution is desirable only if it is configurable which message
types should trigger a CPU redirect and which should not. The reason is
that a hardcoding of message types is unflexible for the introduction of
new message types.

The second solution introduces the risk of new protocols, which are not
related to MLD but uses ICMPv6 and multicast DMAC addresses wrongly
being identified as MLD. We do not suggest a recommended solution in
this case.

The mapping from IP multicast addresses to multicast DMAC addresses
introduces a potentially enormous overlap. The structure of an IPv6 mul-
ticast address is shown in figure 5.  Theoretically 2**80, two to the
power of 80 (128 - 8 - 4 - 4 - 32) unique DIP addresses could map to one
DMAC address. This should be compared to 2**5 in the case of IPv4.

Initial allocation of IPv6 multicast addresses, however, uses only the
lower 32 bits of group ID. This eliminates the address ambiguity for the
time being but it should be noted that the allocation policy may change
in the future.

       |   8    |  4 |  4 |             112 bits                  |
       |11111111|flgs|scop|             group ID                  |
                                Figure 5

In the case of IPv6 forwarding can be made on the basis of DMAC

Christensen, Solensky                                          [Page 12]

RFC DRAFT                                                      June 2001

addresses in the forseable future.

Finally we mention the reserved address range FF0X:0:0:0:0:0:X:X where X
is any value. This range is similar to 224.0.0.X for IPv4 and is
reserved to routing protocols and resource discovery [RFC2375].  In the
case of IPv6 it is suggested that packets in this range are forwarded on
all ports if they are not MLD packets.

5.  Security Considerations

   Security considerations for IGMPv3 are accounted for in [IGMPv3].
   The introduction of IGMP snooping switches adds the following consid-
   erations with regard to IP multicast.

   The exclude source failure which could cause traffic from sources
   that are 'black listed' to reach hosts that have requested otherwise.
   This can also occur in certain network topologies without IGMP snoop-

   It is possible to generate packets which make the switch wrongly
   believe that there is a multicast router on the segment on which the
   source is attached. This will potentially lead to excessive flooding
   on that segment.  The authentication methods discussed in [IGMPv3]
   will also provide protection in this case.

   IGMP snooping switches which rely on the IP header of a packet for
   their operation and which do not validate the header checksum poten-
   tially will forward packets on the wrong ports. Even though the IP
   headers are protected by the ethernet checksum this is a potential

   Generally though, it is worth to stress that IP multicast must so far
   be considered insecure until the work of for example the suggested
   Multicast Security (MSEC) working group or similar is completed or at
   least has matured.

6.  References

     [BRIDGE]  IEEE 802.1D, "Media Access Control (MAC) Bridges"

     [CISCO]   Cisco Tech Notes, "Multicast In a Campus Network: CGMP
               and IGMP snooping", http://www.cisco.com/warp/pub-

Christensen, Solensky                                          [Page 13]

RFC DRAFT                                                      June 2001

     [IANA]    Internet Assigned Numbers Authority, "Internet Multicast
               Addresses", http://www.isi.edu/in-notes/iana/assign-

     [IGMPv3]  Cain, B., "Internet Group Management Protocol, Version
               3", draft-ietf-idmr-igmp-v3-06.txt, November 2000

               Crawford, M., "Transmission of IPv6 Packets over Ethernet
               Networks", RFC2464, December 1998.

     [MLDv2]   Vida, R., "Multicast Listener Discovery Version 2 (MLDv2)
               for IPv6", draft-vida-mld-v2-00.txt, February 2001.

     [MRDISC]  Biswas, S. "IGMP Multicast Router Discovery", draft-ietf-
               idmr-igmp-mrdisc-06.txt, May 2001.

     [MSOFT]   Microsoft support article Q223136, "Some LAN Switches
               with IGMP Snooping Stop Forwarding Multicast Packets on
               RRAS Startup", http://support.microsoft.com/sup-

     [RFC1112] Deering, S., "Host Extensions for IP Multicasting", RFC
               1112, August 1989.

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

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

     [RFC2375] Hinden, R. "IPv6 Multicast Address Assignments", RFC2375,
               July 1998.

Christensen, Solensky                                          [Page 14]

RFC DRAFT                                                      June 2001

7.  Acknowledgements

   We would like to thank Bill Fenner, Yiqun Cai, Edward Hilquist and
   Martin Bak for comments and suggestions on this document.

8.  Author's Addresses:

   Morten Jagd Christensen
   Vitesse Semiconductor Corporation
   Hoerkaer 16
   2730 Herlev
   email: mjc@vitesse.com

   Frank Solensky
   Gotham Networks
   15 Discovery Way
   Acton, MA 01720
   email: fsolensky@GothamNetworks.com

Christensen, Solensky                                          [Page 15]

RFC DRAFT                                                      June 2001

                           Table of Contents

1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
2. IGMP snooping overview  . . . . . . . . . . . . . . . . . . . . .   2
2.1 Problems in older networks . . . . . . . . . . . . . . . . . . .   5
2.2 IGMPv2 snooping and 224.0.0.X  . . . . . . . . . . . . . . . . .   6
2.3 IGMPv3 and IGMPv2 coexistence  . . . . . . . . . . . . . . . . .   8
2.4 Source Specific Joins  . . . . . . . . . . . . . . . . . . . . .   9
3. Snooping Requirements . . . . . . . . . . . . . . . . . . . . . .  10
4. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . . .  11
5. Security Considerations . . . . . . . . . . . . . . . . . . . . .  13
6. References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
7. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
8. Author's Addresses: . . . . . . . . . . . . . . . . . . . . . . .  15

Christensen, Solensky                                          [Page 16]