Designing Redundant Communication Paths in Operational Technology (OT) Environments

Introduction

Redundancy in communication networks for Operational Technology (OT) is not a frivolous pursuit but a foundational principle for resilience and operational continuity. As systems evolve—from early fieldbus topologies to today’s converged IT/OT architectures—the challenge becomes balancing performance, manageability, and robust fault tolerance. In this analysis, we dissect established and emerging approaches to redundant path architecture, underscore the historical evolution of relevant protocols, and provide guidance for secure, manageable deployment in modern industrial environments.

Redundancy: Context and Drivers

The necessity of redundancy in OT networks arises from several practical factors:

• High Availability Requirements: Process automation, manufacturing, and critical infrastructure demand continuous operation, often with recovery time objectives (RTO) measured in seconds or milliseconds.

• Risk Mitigation: Disruptions can propagate physical dangers, not just data loss. Threats originate from hardware failure, software error, human mistake, and increasingly, cyber attack campaigns targeting industrial assets.

Modern OT environments reflect decades of advances: from the early 1980s’ proprietary fieldbuses to converged Ethernet and IP-based systems post-2000, culminating today in complex IT/OT integrations.

Historical Evolution of Redundancy Mechanisms in OT

Fieldbus Era: Proprietary Solutions

Fieldbus protocols such as Profibus, DeviceNet, and Foundation Fieldbus initially introduced basic redundancy, often via hardware-based ring or bus topologies using proprietary failover logic. These solutions, while effective in specific contexts, were difficult to scale and integrate with IT systems.

The Dawn of Ethernet: STP and Industrial Topologies

The adoption of Ethernet in OT in the late 1990s/early 2000s brought with it robust redundancy protocols from IT, most notably Spanning Tree Protocol (STP). However, classic STP, defined in IEEE 802.1D (originally published in 1990), posed challenges:

• Convergence times unsuitable for real-time OT networks

• Lack of application awareness (unicast/multicast traffic treatment)

The industry responded with variants:

• Rapid Spanning Tree Protocol (RSTP, IEEE 802.1w): Improved convergence, but still not deterministic enough for sub-second failovers.

• Multiple Spanning Tree Protocol (MSTP, IEEE 802.1s): Segmentation via multiple logical spanning trees over a single topology.

OT-Specific Innovations: PRP, HSR, and Ring Protocols

By the 2010s, standards addressing OT’s needs emerged:

• Parallel Redundancy Protocol (PRP, IEC 62439-3): Enables devices to send duplicated frames over two separate LANs. Zero recovery time as long as one path remains viable.

• High-availability Seamless Redundancy (HSR, IEC 62439-3): Designed for ring/ring-like topologies, each frame circulates both clockwise and counterclockwise. Instant failover without traffic loss.

• Media Redundancy Protocol (MRP, IEC 62439-2): Targets ring topologies, using a manager switch for ring healing. Typically recovers within 200 ms, but not seamless.

These protocols are tailored for industrial environments, typically running on rugged Ethernet switches and purpose-built endpoints.

Architectural Designs for Redundancy

Star, Ring, and Mesh Topologies

Network topology decisions directly influence the redundancy strategy:

• Star: Centralized, simple but with a single point of failure at the hub. True redundancy requires duplicated central switches.

• Ring: Favored for cost-effective looped resilience. Protocols like MRP and HSR boost effectiveness in process lines or distribution systems.

• Mesh: High active-active redundancy, but greater complexity in management, cost, and possibly unwanted broadcast domains.

Hybrid topologies combining rings and stars are common in large facilities.

Layered Approaches

Layered redundancy—where both the physical and logical layers include redundancy—delivers greater resilience. For example:

• Dual-power supplies and network interface cards (NICs) at endpoints

• Aggregated uplinks using Link Aggregation Control Protocol (LACP, IEEE 802.3ad)

• Multiple routed distributions between process islands and control rooms

Hitless/Seamless Failover: PRP/HSR

Industrial use-cases with zero tolerance for packet loss (e.g., protection relaying in substations, high-speed motion control) benefit from PRP/HSR. These protocols are designed for seamless failover but must be consistently implemented at endpoints and infrastructure to avoid single points of failure.

Challenges in Redundant Design

Broadcast Storms and Loops

Misconfigured or failing redundancy can do more harm than good. Loops in Ethernet cause broadcast storms and network collapse. Employ BPDU Guard, Root Guard, Loop Protection mechanisms, and ensure diligent change control.

Interoperability and Vendor Lock-in

Some industrial redundancy protocols are not universally supported—requiring careful vendor selection or protocol conversion gateways (e.g., PRP RedBox). In multi-vendor scenarios, interoperability testing is essential.

Management and Monitoring Complexity

Redundant architectures increase monitoring and troubleshooting complexity. Modern OT requires:

• Topology-aware monitoring tools capable of visualizing path status and failure

• Centralized log correlation

• Automated failover testing as part of operational routines

Securing Redundant Communication Paths

Segmentation and Compartmentalization

Robust redundancy does not obviate the need for network segmentation. Each redundant path should honor security zones/policies as defined in ISA/IEC 62443 or NIST 800-82 guidance. Use VLANs, firewalls, and access control lists (ACLs) to enforce boundaries even across redundant links.

Authentication and Integrity of Control Protocols

Redundancy protocols are an attack vector if not protected. History teaches us: unprotected STP BPDUs, MRP frames, or PRP/HSR management traffic can be spoofed or manipulated. Deploy network-based authentication, management plane encryption (e.g., TLS-based management), and, where possible, signed control plane messages.

Visibility and Anomaly Detection

Since failover activity can also signal an attack in progress, integrate redundant path monitoring into your security incident & event management (SIEM) workflows. Correlate logs from switches, endpoints, and anomaly detection platforms for comprehensive insight.

Recommendations for Deployment and Continuous Assessment

Stepwise Path to Redundancy

1. Map your topology. Document all physical and logical connections, including implicit paths (e.g., wireless bridges).

2. Select appropriate redundancy protocols. Map protocol capabilities to use-case requirements—prioritizing seamlessness, interoperability, and manageability.

3. Test extensively. Validate failover performance periodically, not just at commissioning, and include negative/failure-path scenarios.

4. Empower IT/OT collaboration. OT engineers and IT security/network teams must work closely to define roles, responsibilities, and escalation paths for failover and incident response.

Continuous Monitoring and Governance

An effective redundancy design is not set-and-forget. Enforce continuous assessment:

• Sustained performance monitoring to identify hidden bottlenecks or failure domains.

• Policy validation to guarantee security and operational boundaries remain intact across topology changes.

• Routine drills and tabletop exercises that simulate both failure and attack scenarios involving redundant communication paths.

Conclusion

Redundant communication paths in OT should be viewed as a dynamic architecture—built upon decades of protocol development and field experience—and not merely a checkbox for compliance. The landscape will continue to evolve: expect greater emphasis on converged IT/OT redundancy strategies, software-defined network overlays, and tighter security integration. The most resilient organizations are those that treat redundancy as an ongoing discipline, integrating IT and OT expertise for robust, secure industrial operations.