21-4
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
78-14064-04
Chapter 21 Configuring IGMP Snooping
Understanding How IGMP Snooping Works
Figure 21-2 Second Host Joining a Multicast Group
Leaving a Multicast Group
These sections describe leaving a multicast group:
• Normal Leave Processing, page 21-4
• Fast-Leave Processing, page 21-5
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 a group-specific 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
Table 21-2 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
To Next Page
Loading...
Need help?
General
