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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 21 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
The switch hardware can distinguish IGMP information packets from other packets for the multicast
group. The information in the table tells the switching engine to send frames addressed to the 224.1.2.3
multicast IP address that are not IGMP packets 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.
Figure 21-2 Second Host Joining a Multicast Group
Multicast-capable router ports are added to the forwarding table for every Layer 2 multicast entry. The
switch learns of such ports through one of these methods:
• Snooping on IGMP queries and Protocol Independent Multicast (PIM) packets
• Statically connecting to a multicast router port with the ip igmp snooping mrouter global
configuration command
Tab le 21-1 IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
224.1.2.3 IGMP 1, 2
Tab le 21-2 Updated IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
224.1.2.3 IGMP 1, 2, 5
Forwarding
table
CPU
Host 1 Host 2 Host 3 Host 4
Router A
PFC VLAN
1
0
234 5
45751