Dell PowerConnect B-RX Configuration Manual page 727

FIGURE 99
The switch snoops for PIM SM join
and prune messages.
The switch detects a source on port1/1
and a receiver for that source's group
on port5/1. It then forwards multicast
data from the source on port1/1
out port5/1 only, which has the receiver.
Without PIM SM traffic reduction, the
switch would forward traffic from the
source out all other ports in the VLAN.
When PIM SM traffic snooping is enabled, the device starts listening for PIM SM join and prune
messages and IGMP group membership reports. Until the device receives a PIM SM join message
or an IGMP group membership report, the device forwards IP multicast traffic out all ports. Once
the 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's source. Since 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's router. The next time the device receives traffic
for 239.255.162.1 from the group's source, the device forwards the traffic only on port 5/1, since
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 result,
the device does not see a join message on behalf of the client. However, since IP multicast traffic
reduction 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 IP multicast traffic reduction 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.
BigIron RX Series Configuration Guide
53-1001986-01
PIM SM traffic reduction in enterprise network
VLAN 2
Port5/1
10.10.10.6
Router sends a
PIM SM join message
Router
for 239.255.162.1.
30.30.30.6
Client
Receiver for Group
239.255.162.1
Configuring IP multicast traffic reduction
VLAN 2
Port1/1
Router
10.10.10.5
VLAN 2
Port7/1
10.10.10.7
Client sends an
IGMP group
Client
membership report
for 239.255.162.69.
Receiver for Group
239.255.162.69
23
Source for Groups
239.255.162.1
239.255.162.69
20.20.20.5
Client
655