Cisco NME-16ES-1G User Guide - Page 37
Destination Address, Type of Packet, Ports, Joining a Multicast Group
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16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series Feature Overview Joining a Multicast Group When a host connected to the switch wants to join an IP multicast group, it sends an IGMP join message, specifying the IP multicast group it wants to join. When the switch receives this message, it adds the port to the IP multicast group port address entry in the forwarding table. Refer to Figure 16. Host 1 wants to join multicast group 224.1.2.3 and multicasts an unsolicited IGMP membership report (IGMP join message) to the group with the equivalent MAC destination address of 0100.5E01.0203. The switch recognizes IGMP packets and forwards them to the CPU. When the CPU receives the IGMP report multicast by Host 1, the CPU uses the information to set up a multicast forwarding table entry as shown in Table 7 that includes the port numbers of Host 1 and the router. Figure 16 Initial IGMP Join Message Cisco router with Ethernet switch network module 1 IGMP Report 224.1.2.3 CPU port Multicast Forwarding Table 88849 2 3 4 5 Host 1 Host 2 Host 3 Host 4 Table 7 IP Multicast Forwarding Table Destination Address 0100.5e01.0203 Type of Packet !IGMP Ports 1, 2 Note that the switch architecture allows the CPU to distinguish IGMP information packets from other packets for the multicast group. The switch recognizes the IGMP packets through its filter engine. This prevents the CPU from becoming overloaded with multicast frames. The entry in the multicast forwarding table tells the switching engine to send frames addressed to the 0100.5E01.0203 multicast MAC address that are not IGMP packets (!IGMP) to the router and to the host that has joined the group. If another host (for example, Host 4) sends an IGMP join message for the same group (Figure 17), the CPU receives that message and adds the port number of Host 4 to the multicast forwarding table as shown in Table 8. Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ 37