Leaving A Multicast Group - Cisco Catalyst 3750 Software Configuration Manual

Understanding IGMP Snooping
Figure 17-2 Second Host Joining a Multicast Group
Table 17-2 Updated IGMP Snooping Forwarding Table
Destination Address
224.1.2.3

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 only to those hosts listed in the forwarding table for that IP multicast
group maintained by IGMP snooping.
When hosts want to leave a multicast group, they can 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.
Catalyst 3750 Metro Switch Software Configuration Guide
17-4
Switching engine
CPU
0
Forwarding
table
2 3
Host 1 Host 2
Router A
1
VLAN
4 5
Host 3 Host 4
Type of Packet Ports
IGMP 1, 2, 5
Chapter 17 Configuring IGMP Snooping and MVR
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