Cisco 2950 Software Configuration Guide - Page 428

Understanding IGMP Snooping Chapter 21 Configuring IGMP Snooping and MVR Note that 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 IGMP packets to only the switch 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 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. Note that because the forwarding table directs IGMP messages to only the CPU, 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. Any unknown multicast traffic is flooded to the VLAN and sent to the CPU until it becomes known. Figure 21-2 Second Host Joining a Multicast Group Router A 1 CPU Switching engine 0 VLAN 45751 Forwarding table 2 3 4 5 Host 1 Host 2 Host 3 Host 4 Table 21-2 Updated IGMP Snooping Forwarding Table Destination Address 0100.5exx.xxxx 0100.5e01.0203 Type of Packet IGMP !IGMP Ports 0 1, 2, 5 Leaving a Multicast Group The router sends periodic multicast general queries and the switch forwards these queries through all ports in the VLAN. Interested hosts respond to the queries. If at least one host in the VLAN wishes to receive multicast traffic, the router continues forwarding the multicast traffic to the VLAN. The switch forwards multicast group traffic to only those hosts listed in the forwarding table for that Layer 2 multicast group. 21-4 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 78-11380-10