Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 413

Chapter 27 Configuring IGMP Snooping for IPv4 Multicast Traffic Understanding How IGMP Snooping Works Figure 27-1 Initial IGMP Join Message Router A CPU PFC 0 1 IGMP report 224.1.2.3 VLAN 45750 Forwarding table 2 3 4 5 Host 1 Host 2 Host 3 Host 4 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 27-1, that includes the port numbers of Host 1, the multicast router, and the switch internal CPU. Table 27-1 IGMP Snooping Forwarding Table Destination Address 0100.5exx.xxxx 0100.5e01.0203 Type of Packet IGMP !IGMP Ports 0 1, 2 The switch 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 27-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table as shown in Table 27-2. 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. OL-11439-03 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY 27-3