In the traditional access/aggregation/core network architecture, the Layer 2 / Layer 3 boundary is at the aggregation switches. This means that all racks connected to the same set of aggregation switches have access to the same VLANs. There is no need for an edge cluster because the edge VM connecting the VLAN with VXLAN based networks can run on any rack. In vCloud Director, if the external networks (VLANs) are trunked to aggregation switches, edge placement is not a concern. The set of racks (clusters) connected to the same aggregation domain usually maps to a vCloud Director provider VDC. The transport zone is then identical to the aggregation domain. The drawback of this design is that provider VDCs cannot span multiple aggregation domains.