Teams should treat overlay routing as a targeted path, not a blanket default route. The goal is to send only traffic for approved private IPs through the overlay while leaving normal internet traffic on the local network path. That preserves performance, limits blast radius, and keeps the design aligned to specific access rules rather than broad interception.
Design overlay routing as a scoped policy, not a full-tunnel default
An overlay network should carry only the traffic that genuinely needs private-path access. For device-to-device use, that means routing approved private IP ranges or explicit destinations through the overlay, while leaving ordinary internet traffic on the local path. This keeps routing intent narrow, reduces unnecessary encapsulation, and avoids turning the overlay into a hidden transit layer for everything.
The practical design question is whether the overlay is being used to extend reach to protected endpoints or to replace the device’s normal network path. In almost every access-control design, the first is safer and easier to reason about. A targeted route table is also easier to troubleshoot because failures stay confined to the protected destination set instead of affecting all traffic.
A good test is whether the traffic would still make sense if the overlay disappeared. If the answer is yes for public internet traffic, that traffic should not depend on the overlay. Reserve the overlay for the private subnets, services, or peer addresses that are actually part of the device-to-device access model.
Keep path selection aligned to access scope and performance
Routing only the approved destinations through the overlay preserves the performance characteristics of the local network. It avoids sending large volumes of unrelated traffic through an extra encapsulation layer, which can add latency, complicate MTU handling, and create confusing bottlenecks that have nothing to do with the protected access requirement.
It also keeps the design aligned to policy. If the access rule says a device may reach a limited set of private resources, the route should reflect that exact allowance rather than broadly intercepting all traffic. That alignment matters because routing is not just a transport choice, it is part of the enforcement boundary for the access model.
For teams designing this well, route scope, destination scope, and policy scope should all describe the same thing. When those three diverge, teams usually see either overreach, where too much traffic is pulled into the overlay, or underreach, where intended private access fails in subtle ways.
Design for clean failure, clear exception handling, and selective expansion
The safest overlay designs are the ones that fail narrowly. If the overlay path is unavailable, only the private destinations that depend on it should be affected, while ordinary internet access continues normally. That makes outages easier to diagnose and limits the blast radius of routing mistakes or control-plane problems.
This also gives teams a cleaner path for exceptions. If a new private service needs overlay access, add only that route or prefix and validate it against the same policy model. Avoid expanding the route set just because it is operationally convenient, because each added destination increases the number of things that can be misrouted or unintentionally exposed.
Selective expansion is especially important when private networks are segmented by environment, function, or sensitivity. Treat each new route as a deliberate access decision, not a network convenience. That discipline is what keeps device-to-device access from becoming a broad implicit trust channel.
Risk and Threat Considerations
Overlay routing can create exposure when teams make the overlay the default path or over-broaden the route set. That can pull unrelated traffic into a protected tunnel, increase the impact of a routing error, and make it harder to see whether access is limited to the intended private destinations.
Failure mechanism: Overly broad routes, overly permissive prefixes, or default-route tunneling can send traffic through the overlay that was never meant to be governed by the access policy, increasing blast radius and making misconfiguration harder to detect.
Impact: Misrouted traffic can degrade performance, complicate incident triage, and widen the practical effect of a compromise or configuration mistake by exposing more traffic to the overlay than the design intended.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
NIST SP 800-53 Rev 5, NIST CSF 2.0 and CIS Controls v8 set the technical controls, while ISO/IEC 27001:2022 defines the regulatory obligations.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST SP 800-53 Rev 5 | AC-4 — Information Flow Enforcement | Overlay routing should enforce which destinations traverse the private path. |
| Recommendation — Restrict overlay routes to approved private destinations and prevent default-path tunneling. | ||
| NIST CSF 2.0 | PR.AA-05 — Managed Access | Scoped overlay access depends on limiting who and what can reach protected resources. |
| Recommendation — Limit device-to-device reachability to explicitly approved private targets. | ||
| ISO/IEC 27001:2022 | A.8.20 — Network security | Overlay routing is a network-security design choice that shapes traffic paths and exposure. |
| Recommendation — Document and enforce route scopes so overlay traffic stays limited to intended services. | ||
| CIS Controls v8 | CIS-12 — Network Infrastructure Management | Overlay routing is a network infrastructure control that benefits from tight configuration management. |
| Recommendation — Maintain explicit route inventories and review changes to overlay paths. | ||
Practitioner Guidance
What to verify: Confirm that only the approved private IP ranges or destination prefixes resolve through the overlay, and that normal internet-bound traffic still exits locally. If you cannot describe the route scope in one sentence, the policy is probably too broad.
What good looks like: A device can reach the intended private resources through the overlay, but its general browsing, software updates, and other ordinary traffic remain on the local path. That separation should be visible in both configuration and troubleshooting behavior.
Practitioner takeaway: Treat routing as an access decision with performance consequences, not as a universal transport shortcut, and keep the overlay as narrow as the protected use case allows.
Related resources from NHI Mgmt Group
- How should security teams decide whether JIT access is safe for non-human identities?
- How should security teams design device trust for web applications when browsers cannot access hardware-backed keys directly?
- How should security teams implement network access controls when supporting compliance and device governance across a growing environment?
- How should security teams handle device posture and network access when remote devices need to connect from unmanaged environments?
Deepen Your Knowledge
Reviewed and updated by the NHIMG editorial team on September 25, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org
