IGMP Snooping 64
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
•
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.
FastLeave
When the switch with IGMP Snooping enabled receives an IGMPv2 leave message, it sends a Group-Specific
Query to determine if any other devices in the same group (and on the same port) are still interested in the
specified multicast group traffic. The switch removes the affiliated port from that particular group, if the following
conditions apply:
•
If the switch does not receive an IGMP Membership Report message within the query-response-interval
•
If no multicast routers have been learned on that port.
With Fastleave enabled on the VLAN, a port can be removed immediately from the port list of the group entry when
the IGMP Leave message is received, unless a multicast router was learned on the port.
Enable FastLeave only on VLANs that have only one host connected to each physical port.
