Cisco Catalyst 2975 Software Configuration Manual page 556

Understanding IGMP Snooping
Router A sends a general query to the switch, which forwards the query to ports 2 through 5, which are
all members of the same VLAN. Host 1 wants to join multicast group 224.1.2.3 and multicasts an IGMP
membership report (IGMP join message) to the group. The switch CPU uses the information in the IGMP
report to set up a forwarding-table entry, as shown in
connected to Host 1 and the router.
Table 22-1
Destination Address
224.1.2.3
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
as shown in
the message is not flooded to other ports on the switch. Any known multicast traffic is forwarded to the
group and not to the CPU.
Figure 22-2
Forwarding
Table 22-2
Destination Address
224.1.2.3
Catalyst 2975 Switch Software Configuration Guide
22-4
IGMP Snooping Forwarding Table
Type of Packet
IGMP
22-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table
Table 22-2. Note that because the forwarding table directs IGMP messages only to the CPU,
Second Host Joining a Multicast Group
Router A
1
PFC
CPU
0
table
2 3
Host 1 Host 2
Updated IGMP Snooping Forwarding Table
Type of Packet
IGMP
Table 22-1, that includes the port numbers
Ports
1, 2
VLAN
4 5
Host 3 Host 4
Ports
1, 2, 5
Chapter 22 Configuring IGMP Snooping and MVR
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