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Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
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Chapter 21 Configuring IGMP Snooping
Understanding How IGMP Snooping Works
Figure 21-1 Initial IGMP Join Message
Multicast 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 with the equivalent MAC destination address of
0x0100.5E01.0203. 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 21-1, that includes
the port numbers of Host 1, the multicast router, and the switch internal CPU.
The router hardware can distinguish IGMP information packets from other packets for the multicast
group. The first entry in the table tells the switching engine to send only IGMP packets to the CPU. This
prevents the CPU from becoming overloaded with multicast frames. The second entry tells the switching
engine to send frames addressed to the 0x0100.5E01.0203 multicast MAC address that are not IGMP
packets (!IGMP) to the multicast 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 21-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table
as shown in Table 21-2. Because the forwarding table directs IGMP messages only to the CPU, the
message is not flooded to other ports. 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
Switching engine
1
0
2345
45750
Table 21-1 IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
0100.5exx.xxxx IGMP 0
0100.5e01.0203 !IGMP 1, 2
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