Enabling IGMP Immediate-Leave
If the querier does not receive a response to a group-specific or group-and-source query, it sends
another (querier robustness value). Then, after no response, it removes the group from the outgoing
interface for the subnet.
IGMP immediate leave reduces leave latency by enabling a router to immediately delete the group
membership on an interface after receiving a Leave message (it does not send any group-specific or
group-and-source queries before deleting the entry).
• Configure the system for IGMP immediate leave.
ip igmp immediate-leave
• View the enable status of the IGMP immediate leave feature.
EXEC Privilege mode
show ip igmp interface
View the enable status of this feature using the command from EXEC Privilege mode, as shown in the
example in Selecting an IGMP Version.
IGMP Snooping
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.
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 virtual local area network
(VLAN) by default, even though there may be only some interested hosts, which is a waste of bandwidth.
If you enable IGMP snooping on a VLT unit, 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 uses IP multicast addresses not MAC addresses.
• IGMP snooping reacts to spanning tree protocol (STP) and multiple spanning tree protocol (MSTP)
topology changes by sending a general query on the interface that transitions to the forwarding state.
• If IGMP snooping is enabled on a PIM-enabled VLAN interface, data packets using the router as an
Layer 2 hop may be dropped. To avoid this scenario, Dell Networking recommends that users enable
IGMP snooping on server-facing end-point VLANs only.

Configuring IGMP Snooping

Configuring IGMP snooping is a one-step process. To enable, view, or disable IGMP snooping, use the
following commands.
There is no specific configuration needed for IGMP snooping with virtual link trunking (VLT). For
information about VLT configurations, refer to Virtual Link Trunking (VLT).
• Enable IGMP snooping on a switch.
CONFIGURATION mode
ip igmp snooping enable
Internet Group Management Protocol (IGMP) 393