A packet is routed between two endpoints on different Cisco ACI leaf switches. Which VXLAN VNID is applied to the packet?

FD (Forwarding Domain) is not a standard VXLAN VNID type in Cisco ACI; VXLAN VNIDs are primarily associated with Bridge Domains (BDs) for Layer 2 and VRFs for Layer 3.

EPG (Endpoint Group) is a grouping of endpoints that share common policies, but it does not directly correspond to a VXLAN VNID used for encapsulation.

In Cisco ACI, when IP packets are routed between different subnets across the fabric, the VXLAN Network Identifier (VNID) used for encapsulation represents the Virtual Routing and Forwarding (VRF) instance. This VRF VNID, often referred to as a Layer 3 VNID, identifies the specific routing context for the packet as it traverses the leaf and spine switches.

BD (Bridge Domain) VNIDs are used for Layer 2 bridging traffic within the same subnet, but not for Layer 3 routed traffic between different subnets, as implied by "routed between two endpoints".