Cisco IE 4000, Software Configuration Manual

Page: 431 / 1066

427
Configuring IGMP Snooping and MVR
Information About IGMP Snooping and MVR
Figure 65 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 the table below, 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 66 on
page 428 ), the CPU receives that message and adds the port number of Host 4 to the forwarding table as shown below.
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.
Destination Address Type of Packet Ports
224.1.2.3 IGMP 1, 2
Forwarding
table
CPU
Host 1 Host 2 Host 3 Host 4
Router A
IGMP report 224.1.2.3
VLAN
PFC
1
0
2 3 4 5
45750