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Understanding IGMP Snooping
Router A sends a general query to the switch, which forwards the query to ports 2 through 5, 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. When the CPU receives the IGMP report multicast by Host 1,
the CPU uses the information in the IGMP report to set up a forwarding-table entry, as shown in
Table
Table 23-1 IGMP Snooping Forwarding Table
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 23-2 Second Host Joining a Multicast Group
Forwarding
Table 23-2 Updated IGMP Snooping Forwarding Table
Destination Address
224.1.2.3
Catalyst 3750 Switch Software Configuration Guide
23-4
23-1, that includes the port numbers connected to Host 1 and the router.
Type of Packet
IGMP
23-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table
Table
23-2. Note that because the forwarding table directs IGMP messages to only the CPU,
Router A
Switching engine
CPU
0
table
2
3
Host 1
Host 2
Type of Packet
IGMP
Ports
1, 2
1
VLAN
4
5
Host 3
Host 4
Ports
1, 2, 5
Chapter 23
Configuring IGMP Snooping and MVR
78-16180-02
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