Spanning Tree Protocols: Inefficiency in Ring Topologies

[Hamed Fazel]

The Spanning Tree Protocol (STP) family, which includes STP, RSTP (Rapid Spanning Tree Protocol), and MSTP (Multiple Spanning Tree Protocol), has been essential for loop protection in networks. However, are these protocols efficient for use in ring topologies?

Challenges of Spanning Tree Protocols in Ring Topologies

In mission-critical industries and large-area networks, using a ring topology is a common way to connect switches for redundancy and coverage. Consider a scenario in a port environment, where the goal is to monitor the perimeter with network cameras (CCTV). In this case, two or more cameras are typically connected to access switches. These switches are then interconnected in a chain-like structure using fiber optics to cover the entire area. The fiber loop ultimately returns to a distribution switch, forming a ring topology for fault tolerance and efficient monitoring.

Ring Topology

In this scenario, the distribution switch often acts as the root bridge, and RSTP blocks a port on a secondary switch (e.g., SW5) to prevent loops. However, if you need to change the blocked port location for traffic engineering, you must adjust the port priority across switches. This manual process is complicated and time-consuming. While MSTP provides more control by allowing VLAN-based path selection, it does not completely address all the challenges of ring topologies.

Limitations of RSTP in Ring Topologies

Despite RSTP offering faster convergence compared to legacy STP, it still presents several drawbacks:

1. Unpredictable Failover and Recovery Times: While RSTP generally offers recovery times around 50ms, these times are inconsistent, leading to unpredictable network behavior during failures.

2. Limited Network Size: RSTP supports a bridge diameter of up to 40 devices. For large ring networks, this restriction can hinder scalability.

3. Suboptimal for Ring Topologies: RSTP is primarily designed for tree-based topologies (e.g., star and extended star networks), which limits its efficiency in ring topologies where loop prevention and rapid failover are critical.

Proprietary Protocol Solutions for Ring Topologies

For many years, there was no standardized solution for redundant LANs capable of providing recovery times suitable for industrial Ethernet applications. As a result, several vendors developed proprietary protocols to address the unique needs of ring network topologies. These protocols provide fast failover, loop protection, and resilient recovery mechanisms tailored for such environments.

Recommendations for Ring Topology Protocols

To overcome the limitations of the Spanning Tree family, several alternative protocols have been developed, providing efficient loop protection and fast failover for ring topologies. Below are some of the most suitable options:

1. ERPS (Ethernet Ring Protection Switching - G.8032)

   • Standard: ITU-T
   • Recovery Time: < 50ms
   • Use Case: Metro Ethernet, service provider networks.

2. REP (Resilient Ethernet Protocol - Cisco Proprietary)

   • Recovery Time: < 50ms
   • Use Case: Industrial Ethernet, enterprise networks.

3. MRP (Media Redundancy Protocol - IEC 62439-2)

   • Standard: IEC 62439-2
   • Recovery Time: ~200ms or faster
   • Use Case: Industrial Ethernet, automation, manufacturing.

4. EAPS (Ethernet Automatic Protection Switching - Extreme Networks Proprietary)

   • Recovery Time: < 50ms
   • Use Case: Metro Ethernet, large enterprises.

5. HiperRing (Hirschmann Proprietary)

   • Recovery Time: Sub-10ms
   • Use Case: Industrial automation, power utilities.

6. Turbo Ring (Moxa Proprietary)

   • Recovery Time: As low as 20ms
   • Use Case: Industrial automation, power, transportation.

7. S-Ring (Korenix Proprietary)

   • Recovery Time: 5ms for up to 250 switches
   • Use Case: Industrial automation, power grids.

8. DT-Ring (Kyland Proprietary)

   • Recovery Time: ~20ms
   • Use Case: Industrial automation, power distribution.

9. RapidRing (Contemporary Controls Proprietary)

   • Recovery Time: Sub-20ms
   • Use Case: Transportation systems, industrial automation.

10. eRSTP (Enhanced RSTP - Siemence Proprietary)

    • Enhancement of RSTP
    • Recovery Time: < 50ms
    • Use Case: Industrial Ethernet networks, factory automation.

Conclusion

While the Spanning Tree family of protocols is essential for loop prevention, its performance in ring topologies is not optimal, particularly in industrial and mission-critical networks. Several proprietary ring protocols offer enhanced recovery times, loop protection, and network resilience. In the subsequent sections, we will explore each protocol in more detail and discuss their benefits in addressing the challenges of ring topologies.