We present an active measurement study of the routing dynamics induced by AS path prepending, a common method for controlling the inbound traffic of a multi-homed AS by manipulating the route's AS-path attribute. Unlike other inter-domain inbound traffic engineering methods, AS path prepending not only provides network resilience but does not increase routing table size. Unfortunately, ISPs often perform prepending on a trail-and-error basis, which can induce a large amount of routing dynamics into the global network. We study these effects by actively injecting prepended routes into the network from the RIPE NCC beacons and observing the effects of these routing changes. Implementation results show that a small number of ASes are responsible for large amounts of route changes introduced by prepending. With this active measurement study, we can reveal some policies to prepending and tie-breaking decisions of upstream ASes which are useful for further predicting the behavior of prepending.