468 | Internet Group Management Protocol (IGMP)w w w . d e l l . c o m | s u p p o r t . d e l l . c o mIGMP SnoopingMulticast packets are addressed with multicast MAC addresses, which represent a group of devices, ratherthan one unique device. Switches forward multicast frames out of all ports in a VLAN by default, eventhough there may be only some interested hosts, which is a waste of bandwidth. IGMP Snooping enablesswitches to use information in IGMP packets to generate a forwarding table that associates ports withmulticast groups so that when they receive multicast frames, they can forward them only to interestedreceivers.If IGMP snooping is enabled on aVLT unit, the IGMP snooping dynamically learned groups and multicastrouter ports are made to learn on the peer by explicitly tunneling the received IGMP control packets.IGMP Snooping Implementation Information• IGMP Snooping on FTOS uses IP multicast addresses not MAC addresses.• IGMP Snooping is supported on all S-Series stack members.• IGMP Snooping reacts to STP and MSTP topology changes by sending a general query on theinterface that transitions to the forwarding state.Configuring IGMP SnoopingConfiguring IGMP Snooping is a one-step process. That is, enable it on a switch using the command ipigmp snooping enable from CONFIGURATION mode. View the configuration using the command showrunning-config from CONFIGURATION mode, as shown in the following example. You can disablesnooping on for a VLAN using the command no ip igmp snooping from INTERFACE VLAN mode.There is no specific configuration needed for IGMP Snooping in conjunction with VLT.FTOS(conf)#ip igmp snooping enableFTOS(conf)#do show running-config igmpip igmp snooping enableFTOS(conf)#Related Configuration Tasks• Enabling IGMP Immediate-leave• Disabling Multicast Flooding• Specifying a Port as Connected to a Multicast Router• Configuring the Switch as Querier
