By default, a device does not examine the IP multicast information in a packet. Instead, the device forwards the packet out all ports except the port that received the packet. In some networks, this method causes unnecessary traffic overhead. For example, if the device is attached to only one group source and two group receivers, but has devices attached to every port, the device forwards group traffic out all ports in the same broadcast domain except the port attached to the source, even though there are only two receivers for the group.
PIM Sparse Mode (PIM-SM) traffic snooping eliminates the superfluous traffic by configuring the device to forward IP multicast group traffic only on the ports that are attached to receivers for the group.
PIM-SM traffic snooping requires IP multicast traffic reduction to be enabled on the device. IP multicast traffic reduction configures the device to listen for IGMP messages. PIM-SM traffic snooping provides a finer level of multicast traffic control by configuring the device to listen specifically for PIM-SM Join and Prune messages sent from one PIM-SM router to another through the device.
Note
This snooping feature applies only to PIM-SM version 2 (PIM v2).A bridge domain is a set of different types of service endpoints, such as pseudowire and VxLAN tunnel, grouped into one broadcast domain that allows any-to-any bridging. A Layer 3 VE interface can be bound to a bridge domain. Because PIM-SM can be configured on the VE, PIM-SM snooping is supported on a bridge domain.
Source-Specific Multicast (SSM) is required only at the last-hop router (LHR) and is not required for the intermediate switch or router. At the LHR, if the SSM map is enabled in the IGMP, the v2 report in the SSM group range is converted to (S,G) join and sent to PIM. If a host sends a v3 report in the same SSM range, and the router is v3 enabled with an SSM map configured on this router, then this report is dropped. From the PIM snooping perspective, the (S,G) join is only created in the software.