usdPimMRouteRouteMask 1.3.6.1.4.1.4874.2.2.43.1.1.12.1.17

The mask of the route being used to do RPF check for this entry. It is set to 255.255.255.255 (with usdPimMRouteRoute set to 0.0.0.0) if there is no route available.