Foundry Configuration Guide for the FESX, FSX, and FWSX
18 - 6
© Foundry Networks, Inc.
December 2005
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.
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 feature also requires the source and the
downstream router to be on different IP sub-nets, as shown in Figure 18.1.
Figure 18.2 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).
NOTE:
This example assumes that the devices are actually FastIron devices running Layer 2 Switch software.
Содержание FastIron Edge Switch X424
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