Leaving A Multicast Group; Immediate-Leave Processing - Cisco Catalyst 2950 Software Configuration Manual

Understanding IGMP Snooping
Figure 16-2 Second Host Joining a Multicast Group
Table 16-2 Updated Multicast Forwarding Table
Destination Address
0100.5e01.0203

Leaving a Multicast Group

The router sends periodic IP multicast general queries, and the switch responds to these queries with one
join response per MAC multicast group. As long as at least one host in the VLAN needs multicast traffic,
the switch responds to the router queries, and the router continues forwarding the multicast traffic to the
VLAN. The switch only forwards IP multicast group traffic to those hosts listed in the forwarding table
for that IP multicast group.
When hosts need to leave a multicast group, they can either ignore the periodic general-query requests
sent by the router, or they can send a leave message. When the switch receives a leave message from a
host, it sends out a group-specific query to determine if any devices behind that interface are interested
in traffic for the specific multicast group. If, after a number of queries, the router processor receives no
reports from a VLAN, it removes the group for the VLAN from its multicast forwarding table.

Immediate-Leave Processing

IGMP snooping Immediate-Leave processing allows the switch to remove an interface that sends a leave
message from the forwarding table without first sending out 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 in use simultaneously.
Catalyst 2950 Desktop Switch Software Configuration Guide
16-4
Router A
Catalyst 2950 switch
CPU
0
CAM
Table
2 3
Host 1 Host 2
Type of Packet
!IGMP
1
4 5
Host 3 Host 4
Ports
1, 2, 5
Chapter 16 Configuring IGMP Snooping and MVR
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