Handbook

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IGMP Snooping
Introduction
IGMP Snooping allows the switch to forward multicast traffic only to those ports that request it. IGMP
Snooping prevents multicast traffic from being flooded to all data ports. The switch learns which server
hosts are interested in receiving multicast traffic, and forwards it only to ports connected to those servers.
The following topics are discussed in this chapter:
Overview
IGMPv3
FastLeave
IGMP Filtering
Static Multicast Router
IGMP Snooping Configuration example
Overview
Internet Group Management Protocol (IGMP) is used by IP Multicast routers to learn about the existence
of host group members on their directly attached subnet (see RFC 2236). The IP Multicast routers get this
information by broadcasting IGMP Query Reports and listening for IP hosts reporting their host group
memberships. This process is used to set up a client/server relationship between an IP Multicast source
that provides the data streams and the clients that want to receive the data.
IGMP Snooping conserves bandwidth. With IGMP Snooping, the switch learns which ports are interested in
receiving multicast data, and forwards multicast data only to those ports. In this way, other ports are not
burdened with unwanted multicast traffic.
The switch currently supports snooping for IGMP version 1 and version 2.
The switch can sense IGMP Membership Reports from attached host servers and act as a proxy to set up
a dedicated path between the requesting host and a local IP Multicast router. After the pathway is
established, the switch blocks the IP Multicast stream from flowing through any port that does not
connect to a host member, thus conserving bandwidth.
The client-server path is set up as follows:
An IP Multicast Router (Mrouter) sends Membership Queries to the switch, which forwards them to all
ports in a given VLAN.
Hosts that want to receive the multicast data stream send Membership Reports to the switch, which
sends a proxy Membership Report to the Mrouter.
The switch sets up a path between the Mrouter and the host, and blocks all other ports from
receiving the multicast.
Periodically, the Mrouter sends Membership Queries to ensure that the host wants to continue
receiving the multicast. If the host fails to respond with a Membership Report, the Mrouter stops
sending the multicast to that path.
The host can send a Leave report to the switch, which sends a proxy Leave report to the Mrouter.
The multicast path is terminated immediately.
IGMPv3
IGMPv3 includes new membership report messages to extend IGMP functionality. The switch provides
snooping capability for all types of IGMP version 3 (IGMPv3) Membership Reports, as described in RFC
3376.
IGMPv3 supports Source-Specific Multicast (SSM). SSM identifies session traffic by both source and group
addresses. The switch uses source filtering, which allows hosts to report interest in receiving multicast
packets only from specific source addresses, or from all but specific source addresses.
The switch supports the following IGMPv3 filter modes:
INCLUDE mode: The host requests membership to a multicast group and provides a list of IP
addresses from which it wants to receive traffic.