Dell PowerConnect 8100 User's Configuration Guide - Page 698

Multicast routers send an IGMP query every 60 seconds. This query is intercepted by the switch and forwarded to all ports in the VLAN. All hosts that are members of the group answer that query. The switch intercepts the replies and forwards only one report per group from all of the received responses. In summary: • IGMP snooping controls the flooding/forwarding behavior for multicast groups. Multicast data is flooded in the VLAN until a multicast router port is identified. • IGMP snooping is enabled by default • Multicast filtering is enabled by default • IGMP snooping forwards multicast sources to multicast routers by default • Reserved multicast IP addresses (224.0.0.x) are always flooded to all ports in the VLAN • Unregistered multicast traffic may be flooded in the VLAN by a user configuration option. NOTE: It is strongly recommended that users enable MLD snooping if IGMP snooping is enabled and vice-versa. This is because both IGMP snooping and MLD snooping utilize the same forwarding table and not enabling both may cause unwanted pruning of protocol packets utilized by other protocols, e.g. OSPFv3. IGMP Snooping Querier When PIM and IGMP are enabled in a network with IP multicast routing, the IP multicast router acts as the IGMP querier. However, if the IP-multicast traffic in a VLAN needs to be Layer 2 switched only, an IP-multicast router is not required. The IGMP snooping querier can perform the role of generating IGMP queries that would normally be performed by the multicast router. NOTE: Without an IP-multicast router on a VLAN, you must configure another switch as the IGMP querier so that it can send queries. When IGMP snooping querier is enabled, the querier switch sends out periodic IGMP queries that trigger IGMP report messages from the hosts that want to receive IP multicast traffic. The IGMP snooping feature listens to 698 Configuring L2 Multicast Features