Configuring L2 Multicast Features 705What Is IGMP Snooping?IGMP Snooping is a layer 2 feature that allows the switch to dynamically addor remove ports from IP multicast groups by listening to IGMP join and leaverequests. By "snooping" the IGMP packets transmitted between hosts androuters, the IGMP Snooping feature enables the switch to forward IPmulticast traffic more intelligently and help conserve bandwidth.Based on the IGMP query and report messages, the switch forwards trafficonly to the ports that request the multicast traffic. This prevents the switchfrom broadcasting the traffic to all ports and possibly affecting networkperformance. The switch uses the information in the IGMP packets as theyare being forwarded throughout the network to determine which segmentsshould receive packets directed to the group address.IGMP Snooping QuerierWhen PIM and IGMP are enabled in a network with IP multicast routing, theIP multicast router acts as the IGMP querier. However, if the IP-multicasttraffic in a VLAN needs to be Layer 2 switched only, an IP-multicast router isnot required. The IGMP Snooping Querier can perform the IGMP snoopingfunctions on the VLAN.When the IGMP snooping querier is enabled, the IGMP snooping queriersends out periodic IGMP queries that trigger IGMP report messages from theswitch that wants to receive IP multicast traffic. The IGMP snooping featurelistens to these IGMP reports to establish appropriate forwarding.What Is MLD Snooping?In IPv4, Layer 2 switches can use IGMP snooping to limit the flooding ofmulticast traffic by dynamically configuring L2 interfaces so that multicasttraffic is forwarded to only those interfaces associated with an IP multicastaddress. In IPv6, MLD snooping performs a similar function. With MLDsnooping, IPv6 multicast data is selectively forwarded to a list of ports thatwant to receive the data instead of being flooded to all ports in a VLAN. Thislist is constructed by snooping IPv6 multicast control packets.NOTE: Without an IP-multicast router on a VLAN, you must configure anotherswitch as the IGMP querier so that it can send queries.
