When an IGMP receiver joins a multicast group, PIM on the leaf router first joins the shared tree. After the first packet is received on the shared tree, the router uses the source address information in the packet to immediately switch over to the shortest path tree (SPT).
To guarantee a simple, yet high-performance implementation of PIM-SM, the switch does not support a threshold bit rate in relation to SPT switchover. Intermediate routers (that is, not directly connected IGMP hosts) do not switch over to the SPT until directed to do so by the leaf routers.
Other vendors can offer a configurable threshold, such as a certain bit rate at which the SPT switch-over occurs. Regardless of their implementation, no interoperability issues with the switch result. Switching to and from the shared and shortest path trees is independently controlled by each downstream router. Upstream routers relay joins and prunes upstream hop-by-hop, building the desired tree as they go. Because a PIM-SM compatible router already supports shared and shortest path trees, no compatibility issues arise from the implementation of configurable switchover thresholds.