Techniques are provided for enabling multicast traffic for a Cable Modem Termination System (CMTS) Modular Headend Architecture (MHA) system. The techniques allow a single modular CMTS core Media Access Control (MAC) domain to service a plurality of physical layer devices. When combined with an independent control plane, Downstream External physical layer (PHY) Interface (DEPI), and Upstream External physical layer (PHY) Interface (UEPI), a downstream PHY device becomes a completely independent and scalable network element.