Cisco 7604 Configuration Manual page 463

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Configuring IGMP Snooping for IPv4 Multicast Traffic

Figure 30-1 Initial IGMP Join Message

Multicast router A sends a general query to the router, 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

the port numbers of Host 1, the multicast router, and the router internal CPU.

Table 30-1 IGMP Snooping Forwarding Table

Destination Address

0100.5exx.xxxx

0100.5e01.0203

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

message is not flooded to other ports. Any known multicast traffic is forwarded to the group and not to

Type of Packet

30-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table

30-2. Because the forwarding table directs IGMP messages only to the CPU, the

Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX

IGMP report 224.1.2.3

Understanding How IGMP Snooping Works

30-1, that includes

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