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412 | Internet Group Management Protocol
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IGMP Snooping
Multicast packets are addressed with multicast MAC addresses, which represent a group of devices, rather
than one unique device. Switches forward multicast frames out of all ports in a VLAN by default, even
though there may be only some interested hosts, which is a waste of bandwidth. IGMP Snooping enables
switches to use information in IGMP packets to generate a forwarding table that associates ports with
multicast groups so that when they receive multicast frames, they can forward them only to interested
receivers.
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 the
interface that transitions to the forwarding state.
Configuring IGMP Snooping
Configuring IGMP Snooping is a one-step process. That is, enable it on a switch using the command ip
igmp snooping enable
from CONFIGURATION mode. View the configuration using the command show
running-config
from CONFIGURATION mode, as shown in Figure 19-9. You can disable snooping on for
a VLAN using the command
no ip igmp snooping from INTERFACE VLAN mode.
Figure 19-9. Enabling IGMP Snooping
Related Configuration Tasks
• Enabling IGMP Immediate-leave on page 412
• Disabling Multicast Flooding on page 413
• Specifying a Port as Connected to a Multicast Router on page 413
• Configuring the Switch as Querier on page 413
Enabling IGMP Immediate-leave
Configure the switch to remove a group-port association upon receiving an IGMP Leave message using the
command
ip igmp fast-leave from INTERFACE VLAN mode. View the configuration using the command
show config from INTERFACE VLAN mode, as shown in Figure 19-10.
FTOS(conf)#ip igmp snooping enable
FTOS(conf)#do show running-config igmp
ip igmp snooping enable
FTOS(conf)#