An engineer must design a secondary site to prevent a catastrophic primary site failure. The applications must communicate across sites via the data link layer of the OSI model. The Layer 3 transport

OTV (Overlay Transport Virtualization) is a Cisco proprietary technology designed specifically to extend Layer 2 segments between geographically dispersed data centers over an existing Layer 3 network, allowing applications to maintain their Layer 2 adjacency across sites.

vPC (Virtual Port Channel) provides Layer 2 multipathing within a single data center or between two devices in the same location, but it does not extend Layer 2 connectivity across geographically separated data centers over a Layer 3 transport.

OSPF (Open Shortest Path First) is an Interior Gateway Protocol (IGP) used for Layer 3 routing within an autonomous system; it does not provide Layer 2 connectivity extension across data centers.

IS-IS (Intermediate System to Intermediate System) is an Interior Gateway Protocol (IGP) used for Layer 3 routing within an autonomous system; it does not provide Layer 2 connectivity extension across data centers.

VXLAN EVPN (Ethernet VPN) is a standards-based solution that provides Layer 2 extension over a Layer 3 IP network, with the EVPN control plane efficiently distributing MAC and IP information, making it ideal for stretching Layer 2 networks between data centers.