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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 20 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
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 to only those hosts listed in the forwarding table for that Layer 2
multicast group.
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 out a MAC-based general 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 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 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 12.1(22)EA2 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 12.1(22)EA3 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.
Table 20-2 Updated IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
0100.5exx.xxxx IGMP 0
0100.5e01.0203 !IGMP 1, 2, 5
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