1-4
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
Figure 1-1 Initial IGMP Join Message
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 Table 1-1, that includes the port numbers connected
to Host 1and the router.
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.
Forwarding
table
CPU
Host 1 Host 2 Host 3 Host 4
Router A
IGMP report 224.1.2.3
VLAN
PFC
1
0
2345
45750
Table 1-1 IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
224.1.2.3 IGMP 1, 2
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