Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches
Network Working Group M. Christensen
Request for Comments: 4541 Thrane & Thrane
Category: Informational K. Kimball
Considerations for Internet Group Management Protocol (IGMP)
and Multicast Listener Discovery (MLD) Snooping Switches
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 (C) The Internet Society (2006).
This memo describes the recommendations for Internet Group Management
Protocol (IGMP) and Multicast Listener Discovery (MLD) snooping
switches. These are based on best current practices for IGMPv2, with
further considerations for IGMPv3- and MLDv2-snooping. Additional
areas of relevance, such as link layer topology changes and
Ethernet-specific encapsulation issues, are also considered.
The IEEE bridge standard [BRIDGE] specifies how LAN packets are
'bridged', or as is more commonly used today, switched between LAN
segments. The operation of a switch with respect to multicast
packets can be summarized as follows. When processing a packet whose
destination MAC address is a multicast address, the switch will
forward a copy of the packet into each of the remaining network
interfaces that are in the forwarding state in accordance with
[BRIDGE]. The spanning tree algorithm ensures that the application
of this rule at every switch in the network will make the packet
accessible to all nodes connected to the network.
This behaviour works well for broadcast packets that are intended to
be seen or processed by all connected nodes. In the case of
multicast packets, however, this approach could lead to less
efficient use of network bandwidth, particularly when the packet is
Christensen, et al. Informational [Page 1]
RFC 4541 IGMP and MLD Snooping Switches Considerations May 2006
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 packets 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. In general,
significant bandwidth can be wasted by flooding.
In recent years, a number of commercial vendors have introduced
products 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 utilize information in the upper
level protocol headers as factors to be considered in 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 address.
In the case of IP 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. This is in contrast to normal switch behavior
where multicast traffic is typically forwarded on all interfaces.
Many switch datasheets state support for IGMP snooping, but no
recommendations for this exist today. It is the authors' hope that
the information presented in this document will supply this
The recommendations presented here are based on the following
information sources: The IGMP specifications [RFC1112], [RFC2236] and
[IGMPv3], vendor-supplied technical documents [CISCO], bug reports
[MSOFT], discussions with people involved in the design of IGMP
snooping switches, MAGMA mailing list discussions, and on replies by
switch vendors to an implementation questionnaire.
Interoperability issues that arise between different versions of IGMP
are not the focus of this document. Interested readers are directed
to [IGMPv3] for a thorough description of problem areas.
The suggestions in this document are based on IGMP, which applies
only to IPv4. For IPv6, Multicast Listener Discovery [MLD] must be
used instead. Because MLD is based on IGMP, we do not repeat the
entire description and recommendations for MLD snooping switches.
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