Brocade FastIron Ethernet Switch IP Multicast Configuration Guide - Manual (Page 38)

device receives a join message or group membership report for a group, the device forwards
subsequent traffic for that group only on the ports from which the join messages or IGMP reports were
received.
In this example, the router connected to the receiver for group 239.255.162.1 sends a join message
toward the group source. Because PIM SM traffic snooping is enabled on the device, the device
examines the join message to learn the group ID, then makes a forwarding entry for the group ID and
the port connected to the receiver router. The next time the device receives traffic for 239.255.162.1
from the group source, the device forwards the traffic only on port 5/1, because that is the only port
connected to a receiver for the group.
Notice that the receiver for group 239.255.162.69 is directly connected to the device. As a result, the
device does not see a join message on behalf of the client. However, because IGMP snooping also is
enabled, the device uses the IGMP group membership report from the client to select the port for
forwarding traffic to group 239.255.162.69 receivers.
The IGMP snooping feature and the PIM SM traffic snooping feature together build a list of groups and
forwarding ports for the VLAN. The list includes PIM SM groups learned through join messages as well
as MAC addresses learned through IGMP group membership reports. In this case, even though the
device never sees a join message for the receiver for group 239.255.162.69, the device nonetheless
learns about the receiver and forwards group traffic to the receiver.
The device stops forwarding IP multicast traffic on a port for a group if the port receives a prune
message for the group.
Notice that the ports connected to the source and the receivers are all in the same port-based VLAN
on the device. This is required for the PIM SM snooping feature. The devices on the edge of the
Global Ethernet cloud are configured for IGMP snooping and PIM SM traffic snooping. Although this
application uses multiple devices, the feature has the same requirements and works the same way as
it does on a single device.
The following figure shows another example application for PIM SM traffic snooping. This example
shows devices on the edge of a Global Ethernet cloud (a Layer 2 Packet over SONET cloud). Assume
that each device is attached to numerous other devices such as other Layer 2 Switches and Layer 3
Switches (routers).
IPv4 Multicast Traffic Reduction
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