Refer to the exhibit.

Overlay Transport Virtualization is the appropriate DCI technology for this mixed three-site environment. Cisco OTV is a MAC-in-IP overlay designed specifically to extend selected Layer 2 broadcast domains across an IP transport. It requires IP reachability between OTV edge devices but does not require the internal switching technology to be identical at every site. That property is central here because the exhibit combines different campus and data-center designs, including vPC, VSS, and a FabricPath-style domain. Extending VLAN 172 through OTV preserves Layer 2 adjacency between Client A and the database server, while each site can retain its existing internal control plane. Inter-subnet communication from Client B in VLAN 192 to Client A in VLAN 172 continues through the local and DCI Layer 3 routing design; OTV supplies the required extension for the shared VLAN rather than replacing routing. A back-to-back vPC is a local multichassis port-channel construct and is unsuitable as a three-site DCI architecture. Dark fibre is only physical transport; by itself it supplies neither a scalable overlay control plane nor selective VLAN extension. VXLAN EVPN can provide modern Layer 2 and Layer 3 overlays, but deploying it end to end would require compatible VTEP and EVPN integration across the disparate legacy domains shown. OTV is built to bridge those heterogeneous sites over their existing routed connectivity. Cisco OTV data-center interconnect design guide

================