Leaving A Multicast Group; Immediate Leave; Igmp Report Suppression - Cisco WS-C3750-48PS-S Software Configuration Manual

Network switch
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Chapter 23
Configuring IGMP Snooping and MVR

Leaving a Multicast Group

The router sends periodic multicast general queries, and the switch forwards these queries through all
ports in the VLAN. Interested hosts respond to the queries. If at least one host in the VLAN wishes to
receive multicast traffic, the router continues forwarding the multicast traffic to the VLAN. The switch
forwards multicast group traffic only to those hosts listed in the forwarding table for that IP multicast
group maintained by IGMP snooping.
When hosts want to leave a multicast group, they can silently leave, or they can send a leave message.
When the switch receives a leave message from a host, it sends a MAC-based general query to learn if
any other devices connected to that interface are interested in traffic for the specific multicast group. The
switch then updates the forwarding table for that MAC group so that only those hosts interested in
receiving multicast traffic for the group are listed in the forwarding table. If the router receives no reports
from a VLAN, it removes the group for the VLAN from its IGMP cache.

Immediate Leave

Immediate Leave is only supported on IGMP Version 2 hosts.
The switch uses IGMP snooping Immediate Leave to remove from the forwarding table an interface that
sends a leave message without the switch sending MAC-based general queries to the interface. The
VLAN interface is pruned from the multicast tree for the multicast group specified in the original leave
message. Immediate Leave ensures optimal bandwidth management for all hosts on a switched network,
even when multiple multicast groups are simultaneously in use.
You should only use the Immediate Leave feature on VLANs where a single host is connected to each
Note
port. If Immediate Leave is enabled in VLANs where more than one host is connected to a port, some
hosts might inadvertently be dropped.

IGMP Report Suppression

IGMP report suppression is supported only when the multicast query has IGMPv1 and IGMPv2 reports.
Note
This feature is not supported when the query includes IGMPv3 reports.
The switch uses IGMP report suppression to forward only one IGMP report per multicast router query
to multicast devices. When IGMP router suppression is enabled (the default), the switch sends the first
IGMP report from all hosts for a group to all the multicast routers. The switch does not send the
remaining IGMP reports for the group to the multicast routers. This feature prevents duplicate reports
from being sent to the multicast devices.
If the multicast router query includes requests only for IGMPv1 and IGMPv2 reports, the switch
forwards only the first IGMPv1 or IGMPv2 report from all hosts for a group to all the multicast routers.
If the multicast router query also includes requests for IGMPv3 reports, the switch forwards all IGMPv1,
IGMPv2, and IGMPv3 reports for a group to the multicast devices.
If you disable IGMP report suppression, all IGMP reports are forwarded to the multicast routers.
78-16180-02
Understanding IGMP Snooping
Catalyst 3750 Switch Software Configuration Guide
23-5

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