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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 20 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
When hosts want to leave a multicast group, they can either silently leave, or they can send a leave
message. When the switch receives a leave message from a host, it sends a group-specific query to
determine 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 Processing
Immediate Leave is only supported with IGMP version 2 hosts.
The switch uses IGMP snooping Immediate-Leave processing to remove from the forwarding table an
interface that sends a leave message without the switch sending group-specific 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 processing ensures optimal bandwidth management for all hosts on a
switched network, even when multiple multicast groups are simultaneously in use.
Note You should only use the Immediate-Leave processing feature on VLANs where a single host is
connected to each 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 Configurable-Leave Timer
In Cisco IOS Release and earlier, the IGMP snooping leave time was fixed at 5 seconds. If membership
reports were not received by the switch before the query response time of the query expired, a port was
removed from the multicast group membership. However, some applications require a leave latency of
less than 5 seconds.
In Cisco IOS Release and later, you can configure the time that the switch waits after sending a
group-specific query to determine if hosts are still interested in a specific multicast group. The IGMP
leave response time can be configured from 100 to 5000 milliseconds. The timer can be set either
globally or on a per-VLAN basis. The VLAN configuration of the leave time overrides the global
configuration.
IGMP Report Suppression
Note IGMP report suppression is supported only when the multicast query has IGMPv1 and IGMPv2 reports.
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.