The Layer 2 multicast traffic must flow across the OTV overlay, and to avoid suboptimal head-end replication, a specific mechanism is required to ensure that multicast capabilities of the transport infrastructure can be leveraged.
The idea is to use a set of Source Specific Multicast (SSM) groups in the transport to carry these Layer 2 multicast streams. These groups are independent from the ASM group previously introduced to transport the OTV control protocol between sites.