30-4
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 30 Configuring IGMP Snooping for IPv4 Multicast Traffic
Understanding How IGMP Snooping Works
Figure 30-2 Second Host Joining a Multicast Group
Leaving a Multicast Group
These sections describe leaving a multicast group:
• Normal Leave Processing, page 30-4
• Fast-Leave Processing, page 30-5
Normal Leave Processing
Interested hosts must continue to respond to the periodic general IGMP queries. As long as at least one
host in the VLAN responds to the periodic general IGMP queries, the multicast router continues
forwarding the multicast traffic to the VLAN. When hosts want to leave a multicast group, they can
either ignore the periodic general IGMP queries (called a “silent leave”), or they can send a
group-specific IGMPv2 leave message.
When IGMP snooping receives a group-specific IGMPv2 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. If IGMP snooping does not receive an IGMP Join message in
response to the general query, it assumes that no other devices connected to the interface are interested
in receiving traffic for this multicast group, and it removes the interface from its Layer 2 forwarding table
entry for that multicast group. If the leave message was from the only remaining interface with hosts
interested in the group and IGMP snooping does not receive an IGMP Join in response to the general
Table 30-2 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
PFC
VLAN
1
0
234 5
45751
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