A mechanism to dynamically map a multicast session to a transport tree to reduce flooding of egress routers on the transport tree is provided. A mechanism to reduce the length of time in which transient flooding can occur while the transport tree is being chosen or configured is also provided. The disclosed dynamic mapping mechanisms avoid interruption of an established multicast session. One mechanism disclosed provides for remapping of a multicast session by cloning an original transport tree with which the multicast session is associated, associating the multicast session with the cloned transport tree, and then reconfiguring the cloned transport tree in accord with edge egress routers that have subscribers to that multicast session.