Generally, using the SPT option eliminates unnecessary levels of PIM-SM traffic in a domain. However, in cases where it is necessary to tightly control the paths used by PIM-SM flows to edge switches, disabling SPT maintains the flows through their original C-RPs regardless of whether shorter paths exist.
Syntax:
When the router is the edge router for a receiver requesting to join a particular multicast group, this command enables or disables the capability of the router to convert the group's traffic from the RPT to the SPT.