Comparative Analysis of Cisco REP and Rapid Spanning Tree Protocol (RSTP) for Resiliency and Redundancy in Industrial OT Networks
- Scott McNeil
- Jul 16
- 6 min read
Updated: Sep 4
Introduction
Industrial Operational Technology (OT) networks are foundational to critical infrastructure sectors such as manufacturing and utilities, where continuous operation is essential and network downtime can result in significant operational and financial losses. To address the stringent requirements for high availability, rapid failover, and robust redundancy, two prominent protocols are often deployed: Cisco’s proprietary Resilient Ethernet Protocol (REP) and the standards-based Rapid Spanning Tree Protocol (RSTP, IEEE 802.1w). Both protocols are designed to enhance network resiliency and minimize service interruptions, but they differ in operational principles, compatibility, and best-fit scenarios. This paper provides a comparative analysis of REP and RSTP, examining their suitability for industrial OT environments, resiliency characteristics, redundancy mechanisms, and the added benefits of deploying Cisco REP over EtherChannel. By highlighting the advantages and limitations of each approach, the analysis aims to guide practitioners in selecting the most appropriate protocol for their specific industrial networking requirements.
Protocol Overview
Cisco Resilient Ethernet Protocol (REP)
Cisco Resilient Ethernet Protocol (REP) is a proprietary protocol developed by Cisco to address the need for fast convergence and robust resiliency in industrial Ethernet networks, particularly those using ring and linear topologies. REP operates by organizing network ports into a segment, with each segment consisting of two user-configured edge ports and a series of standard segment ports. To prevent network loops, REP dynamically blocks a single port—known as the Alternate (ALT) port—within the segment. In the event of a link failure, REP rapidly unblocks the previously blocked port, restoring full connectivity and minimizing network disruption. This mechanism enables REP to deliver rapid failover and improved convergence times compared to traditional spanning tree-based solutions, making it well-suited for environments where downtime must be minimized and deterministic recovery is essential.
A key advantage of REP is its support for advanced features such as VLAN load balancing and integration with EtherChannel for added redundancy. REP allows for logical blocking per VLAN, enabling traffic engineering and efficient use of available bandwidth within the segment. The protocol is designed for deployment exclusively on Cisco devices (which include Rockwell Stratix devices) and is supported only on trunk ports, ensuring tight integration with Cisco’s switching platforms. REP uses a local link failure detection mechanism, known as the Link Status Layer (LSL), to establish neighbor relationships and maintain segment integrity without relying on end-to-end polling. While REP excels at handling single link failures quickly, it has limitations: it can only manage a single failed port within a segment, and multiple simultaneous failures may result in significant loss of connectivity. Proper configuration is critical, as misconfiguration can introduce loops or disrupt network operations. Overall, REP provides a robust foundation for constructing resilient, redundant, and high-performance industrial Ethernet networks within Cisco environments.
Rapid Spanning Tree Protocol (RSTP)
Rapid Spanning Tree Protocol (RSTP), standardized as IEEE 802.1w, is a significant enhancement over the original Spanning Tree Protocol (STP), primarily designed to deliver much faster network convergence following topology changes. Unlike classic STP, which can take up to 30–50 seconds to recover from failures, RSTP can achieve convergence in less than a second by employing a more efficient system of port roles and states when configured properly and not left at default settings. RSTP introduces distinct port roles—root, designated, and alternate/backup—that allow switches to rapidly determine the best path for forwarding traffic and to immediately transition ports to the forwarding state when appropriate. This rapid adaptation minimizes network downtime and ensures continuous data flow, which is critical for industrial OT environments where process interruptions are costly.
A key strength of RSTP is its topology-agnostic design, enabling its deployment across a wide range of network architectures including ring, mesh, star, and hybrid configurations. This flexibility makes RSTP highly suitable for diverse and evolving OT networks, where requirements may change over time or where multi-vendor interoperability is essential. As an open standard, RSTP is supported by most major networking vendors, facilitating seamless integration in mixed environments and future-proofing investments in network infrastructure. Its use of standardized Bridge Protocol Data Units (BPDUs) for communication and loop prevention further enhances its reliability and compatibility, making RSTP a foundational protocol for resilient and redundant industrial OT networking.
EtherChannel Integration
A notable feature of Cisco REP is its compatibility with EtherChannel, a technology that aggregates multiple physical links into a single logical connection. When REP is configured over an EtherChannel interface, it enhances redundancy by providing multiple parallel physical paths, thereby increasing bandwidth and reducing the risk of a single point of failure. It is important to note that REP operates on the EtherChannel interface as a whole and not on individual member ports. RSTP also supports EtherChannel, treating the aggregated links as a single logical interface for loop prevention and failover purposes. This ensures that both protocols can leverage link aggregation for improved redundancy and performance.
Comparative Analysis Matrix
Feature / Aspect | Cisco REP | RSTP (IEEE 802.1w) |
Protocol Type | Proprietary (Cisco/Stratix only) | Open standard (multi-vendor) |
Topology Focus | Optimized for ring and linear topologies | Supports any topology (ring, mesh, star, etc.) |
Redundancy & Resiliency | Fast failover in ring; recovers from single link failure within segment | Fast failover (typically <100ms in rings up to 20 switches) |
Convergence Time | Very fast (optimized for industrial rings) | Fast (improved over classic STP, slightly slower than REP in some rings) |
Loop Prevention | Blocks one port in the ring to prevent loops, dynamic port unblocking on failure | Uses BPDUs and port roles to prevent loops |
VLAN Load Balancing | Supported | Supported |
Multi-Vendor Interoperability | No (Cisco & Stratix only) | Yes |
Configuration Complexity | Requires careful configuration; misconfiguration can cause loops | Generally simpler, but requires understanding of port roles |
Multiple Failures | Handles single link failure; multiple failures can disrupt connectivity | Handles multiple failures, but recovery depends on topology |
EtherChannel Support | Supported at the EtherChannel level (not on individual member ports) | Supported on individual ports |
Scalability | Limited to Cisco and Stratix hardware; segment size and features vary by platform | Scales well; limited mainly by hardware performance |
Use Case Suitability | Industrial rings where Cisco or Stratix is the exclusive vendor | Mixed-vendor environments, flexible topologies |
Use Case Recommendations
Cisco REP is ideally suited for:
Industrial ring and linear topologies with Cisco or Stratix devices: REP is purpose-built for environments where all network devices are from Cisco or compatible Stratix product lines, ensuring full feature support and seamless integration.
Applications demanding sub-second failover and minimal operational disruption: REP is engineered for extremely fast convergence—typically less than 50 milliseconds—making it ideal for critical OT systems where downtime must be minimized.
Deployments requiring additional redundancy through EtherChannel aggregation: REP supports operation over EtherChannel interfaces, allowing multiple physical links to be combined for higher bandwidth and improved link resilience.
Networks needing VLAN load balancing and optimized bandwidth utilization: REP enables VLAN-based load balancing, allowing efficient traffic distribution and maximizing the use of available network resources.
Complex industrial environments with multiple interconnected rings or subrings: REP can be configured to support advanced topologies, providing flexibility and scalability for large-scale industrial deployments.
Installations prioritizing centralized management and simplified troubleshooting: REP is compatible with Cisco network management tools, enabling streamlined configuration, monitoring, and rapid fault isolation.
Scenarios where deterministic network recovery and predictable behavior are required: REP’s deterministic failover mechanism ensures that network recovery times are consistent and predictable, which is essential for time-sensitive industrial processes.
RSTP is recommended for:
Multi-vendor industrial environments: RSTP is an open IEEE 802.1w standard, making it ideal for OT networks with switches and devices from multiple manufacturers, ensuring interoperability and futureproofing.
Flexible network topologies: RSTP is topology-agnostic and supports ring, mesh, star, and hybrid architectures, providing adaptability for evolving or complex industrial network designs.
Applications requiring rapid fault recovery: RSTP delivers fast convergence, typically under 1 second in rings of up to 20 switches—making it suitable for mission-critical automation, SCADA, and process control systems where downtime must be minimized.
Networks that require high availability and loop prevention: By automatically blocking redundant paths and swiftly rerouting traffic around failed nodes, RSTP maintains continuous network operation and prevents broadcast storms or loops.
Installations with diverse or growing protocol needs: RSTP works seamlessly with a wide range of industrial Ethernet and IP-based fieldbus protocols (e.g., Modbus-TCP, Profinet, Ethernet/IP), supporting the connectivity of PLCs, IEDs, and sensors to central control systems.
Scenarios demanding standards-based redundancy: RSTP’s international standardization ensures rigorous peer review, compatibility with future Ethernet developments, and consistent behavior across platforms.
Small to medium-sized networks that require dependable, fast convergence: RSTP is particularly effective in networks that prioritize quick recovery and predictable delay, though it may be less suitable for extremely large or heavily loaded topologies.
Conclusion
Both Cisco REP and RSTP offer robust mechanisms for redundancy and resiliency in industrial OT networks. REP provides ultra-fast recovery and is highly effective in Cisco or Stratix ring or linear topologies, especially when combined with EtherChannel for added redundancy. RSTP, as an open standard, delivers comparable convergence speeds, greater flexibility, and multi-vendor compatibility, making it the preferred choice for mixed or evolving OT environments. The selection between REP and RSTP should be guided by network topology, vendor landscape, and perhaps most importantly, specific operational requirements.
References & Sources
Cisco Systems. (2024). Cisco Resilient Ethernet Protocol Configuration Guide.
IEEE. (2001). IEEE Standard for Local and Metropolitan Area Networks: Rapid Reconfiguration of Spanning Tree Protocol (IEEE 802.1w).
Industrial Ethernet Book. (2023). Industrial Network Redundancy Best Practices.
