Flexible datacenters rely on high-bandwidth server-rack fabrics to allocate their distributed computing and storage resources anywhere, anyhow, and anytime demanded. We describe the multicast architecture of a distributed serverrack fabric, which is arranged around a spine-leaf topology and connects 640 Ethernet ports running at 100G. To cope with the immense fabric speed, we resort to hierarchical, tree-based replication, facilitated by specially commissioned fabric-end ports. At each (port-to-port) leg of the tree, a frame copy is forwarded after a request-grant admission phase and is ACKed by the receiver. To save on bandwidth, we use a packet cache in our input-queued switching-nodes, which replicates asynchronously forwarded frames thus tolerating the variable-delay in the admission phase. Because the cache has limited size, we loosely synchronize the multicast subflows to protect the cache from thrashing. We describe our policies for lossy classes, which segregate and provide fair treatment to multicast subflows. Finally, we show that industry-standard Level2 congestion control does not adapt well to one-to-many flows, and demonstrate that the methods that we implement achieve the best performance.