A double-sided virtual port channel (vPC) topology, also known as a back-to-back vPC, requires two distinct vPC domains connected via a single logical port channel. For the LEAF and TOR switches to form this topology, both layers must independently operate as vPC peer devices. The scenario states that LEAF1 and LEAF2 are already configured as vPC peers. To complete the double-sided vPC architecture shown in the exhibit, the engineer must establish a vPC peer link and keepalive connection between TOR1 and TOR2, effectively configuring them as a second vPC domain. This allows all physical links between the LEAF and TOR layers to bundle into a unified vPC (Po1).

A. Adding switches to a fabric is an ACI concept and does not configure the required peer links for a standalone NX-OS double-sided vPC architecture.

B. vPC peering is strictly established between lateral peers in the same domain (e.g., TOR1 to TOR2), never vertically between different layers like LEAF and TOR.

C. Spanning tree protocols like MSTP provide loop prevention but do not establish the required vPC domains, peer links, or peer keepalive messages necessary for double-sided vPCs.

[1] Cisco Systems. "Cisco Nexus 9000 Series NX-OS Interfaces Configuration Guide, Release 9.3(x)." Chapter: Configuring vPCs, Section: vPC Topologies (Double-Sided vPCs).

[2] Cisco Systems. "Cisco Virtual PortChannel Quick Configuration Guide." Section: vPC Topology - Double Sided vPC. Document ID: 118938.