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17-4
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 17 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
Figure 17-2 Second Host Joining a Multicast Group
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
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.
Table 17-2 Updated IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
0100.5exx.xxxx IGMP 0
0100.5e01.0203 !IGMP 1, 2, 5
Forwarding
table
CPU
Host 1 Host 2 Host 3 Host 4
Router A
Switching engine
VLAN
1
0
234 5
45751