Configuration Notes and Limitations for PIM SM Snooping

  • PIM SM snooping applies only to PIM SM version 2 (PIM SM V2).
  • PIM SM traffic snooping is supported on both the switch and router image.
  • IGMP snooping must be enabled on the device that will be running PIM SM snooping. The PIM SM traffic snooping feature requires IGMP snooping.
  • On router images, PIM SM traffic snooping is not supported for the default VLAN.
Note: Use the passive mode of IGMP snooping instead of the active mode. The passive mode assumes that a router is sending group membership queries as well as join and prune messages on behalf of receivers. The active mode configures the device to send group membership queries.
  • All the device ports connected to the source and receivers or routers must be in the same port-based VLAN.
  • The PIM SM snooping feature assumes that the group source and the device are in different subnets and communicate through a router. The source must be in a different IP subnet than the receivers. A PIM SM router sends PIM join and prune messages on behalf of a multicast group receiver only when the router and the source are in different subnet. When the receiver and source are in the same subnet, they do not need the router in order to find one another. They find one another directly within the subnet.

The device forwards all IP multicast traffic by default. Once you enable IGMP snooping and PIM SM traffic snooping, the device initially blocks all PIM SM traffic instead of forwarding it. The device forwards PIM SM traffic to a receiver only when the device receives a join message from the receiver. Consequently, if the source and the downstream router are in the same subnet, and PIM SM traffic snooping is enabled, the device blocks the PIM SM traffic and never starts forwarding the traffic. This is because the device never receives a join message from the downstream router for the group. The downstream router and group find each other without a join message because they are in the same subnet.

Note: If the "route-only" feature is enabled globally on a Layer 3 switch, PIM SM snooping traffic is not supported.