The most cost-effective and secure solution is to configure VPN concentrators inside the OT networks at both sites (Option D). This setup allows encrypted communications between Site A and Site B, enabling controllers at either site to serve as secondary or failover devices for the other. By leveraging VPN tunnels, the organization avoids the expensive and time-consuming process of laying new fiber infrastructure, while still ensuring secure, authenticated, and encrypted connections across sites.
Option A, direct fiber connectivity, provides high performance but is extremely costly and less flexible than VPN solutions. Option B, deploying redundant SCADA controllers at each site, increases hardware, licensing, and management costs while still requiring interconnectivity. Option C, air-gapping the OT network, may improve isolation but would prevent remote failover capabilities, contradicting the requirement for cross-site control.
By implementing VPN concentrators, the organization achieves secure cross-site redundancy, supports operational continuity in case of controller outages, and does so in a cost-effective manner aligned with common OT security practices.