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cannot determine whether the reported multicast group still has active members attached to that port. For
more information about the IGMP report suppression mechanism, see "Configuring IGMP."
Leave message
An IGMPv1 host does not send any leave messages when it leaves a multicast group. The Layer 2 device
cannot immediately update the status of the port that connects to the receiver host. It removes the port
from the associated forwarding entry when the aging timer for the multicast group on the port expires.
For a static member port, this mechanism does not take effect.
An IGMPv2 or IGMPv3 host sends an IGMP leave message to the multicast router when it leaves a
multicast group.
When the switch receives an IGMP leave message on a dynamic member port, the switch looks up the
forwarding table for a matching entry:
• If no match is found, the switch directly discards the IGMP leave message.
• If a match is found but the receiving port is not in the forwarding entry, the switch directly discards
the IGMP leave message.
• If a match is found and the receiving port is in the forwarding entry, the switch forwards the leave
message to all router ports in the VLAN. The switch does not immediately remove the port from the
forwarding entry for that group. Instead, it adjusts the aging timer for the port.
After receiving the IGMP leave message, the IGMP querier resolves the multicast group address in the
message. Then, it sends an IGMP group specific query to the multicast group through the port that
received the leave message.
After receiving the IGMP group specific query, the switch forwards the query through all its router ports
in the VLAN and all member ports of the multicast group. Then, it waits for responding IGMP reports from
the directly connected receivers to check for the existence of members for the multicast group. For the port
that receives the leave message (assuming that it is a dynamic member port), the Layer 2 device also
performs one of the following actions:
• If the port receives an IGMP report before the aging timer expires, the switch restarts the aging timer
for the port.
• If the port does not receive any IGMP reports when the aging timer expires, the switch removes the
port from the forwarding entry for the multicast group.
Protocols and standards
RFC 4541, Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener
Discovery (MLD) Snooping Switches
IGMP snooping configuration task list
Task at a
lance
Configuring basic IGMP snooping functions:
• (Required.) Enabling IGMP snooping
• (Optional.) Specifying the IGMP snooping version
• (Optional.) Setting the maximum number of IGMP snooping forwarding entries
• (Optional.) Configuring parameters for IGMP queries and responses
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