Cisco ME 3400G-2CS - Ethernet Access Switch Software Configuration Manual page 381

Understanding IGMP Snooping
Table 20-1
Destination Address
224.1.2.3
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
as shown in
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 20-2
Forwarding
Table 20-2
Destination Address
224.1.2.3
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
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
20-4
IGMP Snooping Forwarding Table
Type of Packet
IGMP
20-2), the CPU receives that message and adds the port number of Host 4 to the forwarding table
Table 20-2. Note that because the forwarding table directs IGMP messages to only the CPU,
Second Host Joining a Multicast Group
Router A
PFC
CPU
0
table
2 3
Host 1 Host 2
Updated IGMP Snooping Forwarding Table
Type of Packet
IGMP
Ports
1, 2
1
VLAN
4 5
Host 3 Host 4
Ports
1, 2, 5
Chapter 20 Configuring IGMP Snooping and MVR
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