468 | Internet Group Management Protocol (IGMP)
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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.
If IGMP snooping is enabled on a VLT unit, the IGMP snooping dynamically learned groups and multicast
router 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 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 the following example. You can disable
snooping 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 enable
FTOS(conf)#do show running-config igmp
ip igmp snooping enable
FTOS(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
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