26-4
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 26 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
The switch hardware can distinguish IGMP information packets from other packets for the multicast
group. The information in the table tells the switching engine to send frames addressed to the 224.1.2.3
multicast IP address that are not IGMP packets to the router and to the host that has joined the group.
If another host (for example, Host 4) sends an unsolicited IGMP join message for the same group
(Figure 26-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table
as shown in Table 26-2. Note that because the forwarding table directs IGMP messages only to the CPU,
the message is not flooded to other ports on the switch. An
y known multicast traffic is forwarded to the
group and not to the CPU.
Figure 26-2 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 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 a group-specific query to learn 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.
Ta ble 26-2 Updated IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
224.1.2.3 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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