370 | 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.IGMP Snooping Implementation Information• IGMP Snooping on FTOS uses IP multicast addresses not MAC addresses.• IGMP Snooping is not supported on stacked VLANs.• 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 Figure 18-9. You can disable snooping on fora VLAN using the command no ip igmp snooping from INTERFACE VLAN mode.Figure 18-9. Enabling IGMP SnoopingRelated Configuration Tasks• Enabling IGMP Immediate-leave on page 370• Disabling Multicast Flooding on page 371• Specifying a Port as Connected to a Multicast Router on page 371• Configuring the Switch as Querier on page 371Enabling IGMP Immediate-leaveConfigure the switch to remove a group-port association upon receiving an IGMP Leave message using thecommand ip igmp fast-leave from INTERFACE VLAN mode. View the configuration using the commandshow config from INTERFACE VLAN mode, as shown in Figure 18-10.FTOS(conf)#ip igmp snooping enableFTOS(conf)#do show running-config igmpip igmp snooping enableFTOS(conf)#
