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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 28 Configuring IGMP Snooping and MVR
Information About IGMP Snooping and MVR
Router A sends a general query to the switch, which forwards the query t o 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 28-1, 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 engi ne 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 28-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table
as shown in Tabl e 28-2. 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.
Figure 28-2 Second Host Joining a Multicast Group
Tab l e 28-1 IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
224.1.2.3 IGMP 1, 2
Tab l e 28-2 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 3Host 4
Router A
PFC VLAN
1
0
2345
45751