Hello David
The PIM Assert mechanism is primarily used in dense mode where by the nature of dense mode, it is necessary to eliminate duplicate multicast streams. However, it is also useful in some cases in sparse mode.
In PIM Sparse Mode, multicast traffic is only sent to routers that explicitly join the multicast group using PIM Join messages. So, in theory, only one router on a LAN should be forwarding multicast traffic for a given group to that LAN.
However, there are real scenarios where multiple routers may end up forwarding the same multicast stream onto the same multi-access segment, even in sparse mode.
For example, if two routers on the same Ethernet segment both have receivers for the same group, each sends a PIM join towards the RP or source, and each may build its own SPT or share the RPT path, resulting in both routers forwarding downstream to the same shared LAN.
In sparse mode this could also occur if the source is directly connected to the multi-access network, during the switchover from the RPT to SPT, as well as if a receiver moves or if there are IGMP snooping inconsistencies.
So sparse mode does try to avoid duplicate forwarding — but it’s not foolproof.
That’s why PIM Assert should still be used on multi-access networks — to detect and suppress unexpected or transient duplicate forwarding of multicast traffic.
I hope this has been helpful!
Laz