For proper multicast traffic flow in a PIM-SM domain, as a general rule, enable PIM-SM on all interfaces in the network (even if paths exist between all PIM interfaces). Enable PIM on all interfaces because PIM-SM relies on the unicast routing table to determine the path to the RP, BSR, and multicast sources. Ensure that all routers on these paths have PIM-SM enabled interfaces.
The following figure provides an example of this situation. If A is the RP, then initially the receiver receives data from the shared tree path (that is, through switch A).
If the shortest path from C to the source is through switch B, and the interface between C and B does not have PIM-SM enabled, then C cannot switch to the SPT. C discards data that comes through the shared path tree (that is, through A). The simple workaround is to enable PIM on VLAN1 between C and B.
