All you need to do is enable Multicast in the Distribution and patch settings used on the client or for that task.
From What's New in LDMS 9.6 Software Distribution
4. Self-Organizing Multicast built into clients for quicker more efficient multicasting.
With the removal of Targeted Multicasting from the Global Scheduler on the Core, we have switched to a more efficient method called Self-Organizing Multicast. If the Multicast box is checked in the Distribution and Patch settings, then all the client in the task will communicate with other machines in the task on their same subnet, they will elect a machine to be the Multicast Domain Representative (MDR), who will be the sole machine downloading the files from the best location (Peer, Preferred Server, or Source) and will then multicast each file out the other side while the next file is downloading.
Previously the Core would elect the MDR, which depended on whether or not the network allowed subnet directed boradcasts, then the Core would push the files from the source to the MDR and once he had all the files he would begin the multicast. It was a drawn out process that took longer than desired in many cases, but allowed for minimum network traffic across WAN connections.
The new method is faster and more efficient on network communication overall.