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A.never takes place
B.periodically
C.during switchover
D.after switchover
E.before switchover

A.It prevents message looping， Session Advertisement （SA）messages must be RPF checked
B.RPF check should be done against the route to the source S of the corresponding PIM-SM domain
C.RPF check should be done against the route to the Rendez-vous Point of the corresponding PIM-SM domain，which originated the SA
D.RPF checking Session Advertisement （SA） messages will cause message looping