What is the overall assessment of VPNs as an architecture building block?

VPNs are useful, widespread components for extending perimeters beyond physical boundaries, and can connect almost anything (data centres, clients, OT, IoT, cloud) as far as latency and reliability allow. Other overlay technologies exist for special purposes, but check their suitability as break-in and break-out protection before relying on them.

Points to keep in mind:

• The limits are latency and availability: a tunnel over the internet is only as fast and reliable as the internet path.
• Not every overlay is a security tool. Cisco's EVPN VXLAN, a Layer 2 overlay, is an example of an overlay technology built for networking, and whether it keeps intruders out (and data in) must be checked before treating it as a perimeter.
• The perimeter security of every connected area must be fully maintained. A VPN joins the perimeters, so a weak side weakens both.