You can use the clear ip pim commands to reset PIM counters and mappings.
clear
ip pim auto-rp
Use to clear the group-to-RP router mappings that the
router learned through auto-RP.
Specify the IP address of an RP to clear the group-to-RP
mappings for a particular RP. If you do not specify an IP address,
the router clears the group-to-RP mappings on all RP routers learned
through auto-RP.
Use to clear the counters for multicast packet statistics
on all interfaces or a specified interface.
Specify an interface type and specifier, such as atm 3/0,
to clear the counters on that interface. For details about interface
types and specifiers, see Interface Types and Specifiers in JUNOSe Command Reference Guide.
If you do not specify an interface, the router clears
the counters on all interfaces.