In Cisco ACI fabric, when a Layer 2 unknown unicast packet is encountered, the fabric does not drop the packet. Instead, it uses a flooding mechanism to ensure that the packet reaches its potential destination. This is achieved by utilizing the VRF (Virtual Routing and Forwarding) Outer Multicast Group ID associated with the Bridge Domain (BD) to which the unknown destination MAC address belongs. By doing so, Cisco ACI ensures that all endpoints within the BD have an opportunity to receive and process the packet if they are indeed the intended recipient. References: Implementing Cisco Application Centric Infrastructure Official Cert Guide