Filter
Top Products
22-4
Catalyst 2960 and 2960-S Switch Software Configuration Guide
OL-8603-09
Chapter 22 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
Figure 22-1 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 22-1, that includes the port numbers
connected to Host 1 and 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.
If another host (for example, Host 4) sends an unsolicited IGMP join message for the same group
(Figure 22-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table
as shown in Table 22-2. Note that because the forwarding table directs IGMP messages only to the CPU,
the message is not flooded to other ports on the switch. Any known multicast traffic is forwarded to the
group and not to the CPU.
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 22-1 IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
224.1.2.3 IGMP 1, 2