Juniper Networks China Mobile ZTE Nvidia Cerebras Biren Technology Use of multicast in AI Data Centers (AIDC) is getting attention for efficient large-scale distribution of the same data to many receivers, given the collectives like All2All and AllReduce. Emerging technologies like In Network Compute(INC) can also benefit from using in-network-distribution of the packets by offloading the distribution of the flows to the network. Given the bursty nature of short-lived all2all flows, BIER is a very good multicast technology for AIDC because it does not need the per-establishment of the multicast tree states. This document discusses further optimization of BIER use in AIDC or similar deployment scenarios, and updates RFC4604 by specifying an IGMP/MLD extension for sources to report receiver information to the First Hop Routers. The extension can be useful and is only needed when the source cannot impose the BIER encapsulation.