Configuring SCALANCE Switch Ring Redundancy (MRP and HRP) for PCS 7 Networks

Configuring industrial Ethernet ring redundancy using Siemens SCALANCE X managed switches in SIMATIC PCS 7 requires selecting the appropriate protocol: High-Speed Redundancy Protocol (HRP) for legacy optical/electrical rings or international standard Media Redundancy Protocol (MRP according to IEC 62439-2) for PROFINET Plant and Fieldbuses. In an MRP ring configuration, exactly one SCALANCE switch must be configured as the MRP Redundancy Manager (Master) while all other switches, interface modules, and communications processors operate as MRP Clients. Ring ports must be explicitly designated (typically ports 1 and 2), and ring redundancy must be harmonized between the switch Web Based Management (WBM) and STEP 7 Hardware Configuration.

Ring Redundancy Protocols in PCS 7: HRP vs MRP

In industrial automation networks, physical bus or star topologies present single points of failure: a severed Ethernet cable or damaged switch port isolates downstream controllers and distributed I/O stations. To prevent plant shutdowns, SIMATIC PCS 7 utilizes ring topologies governed by deterministic media redundancy protocols:

High-Speed Redundancy Protocol (HRP): Siemens proprietary protocol supporting fast recovery times (< 300 ms for up to 50 switches) across electrical and optical SCALANCE switches. Extensively deployed on legacy Plant Buses.
Media Redundancy Protocol (MRP - IEC 62439-2): Open international standard protocol supporting bumpless recovery times of 200 ms for up to 50 nodes. Mandated for modern PROFINET IO networks interconnecting ET 200SP HA, Compact Field Units (CFU), and S7-400 CPs.

Both protocols prevent catastrophic network broadcast storms by blocking one physical ring port under normal conditions, unblocking it within 200 ms when a cable rupture is detected.

Switch Role Allocation: Redundancy Manager vs Client

Every redundant ring must have a clear hierarchical governance structure:

Node TypeConfigured RoleOperational FunctionAllowed Quantity per RingCore SCALANCE Switch (e.g., XC208 / X308)Redundancy Manager (Master)Continuously transmits test frames; blocks loop; opens alternate path on breakStrictly One (1) per ringStandby SCALANCE SwitchRedundancy Manager (Auto)Automatically assumes Manager role if primary Manager fails (HRP mode)Optional (in HRP mode)Intermediate SCALANCE SwitchesRedundancy ClientForwards ring test frames; reports local port link status to ManagerUnlimited (up to 50 nodes)Peripheral Devices (ET 200, CP 443-1)MRP ClientParticipates in ring via onboard dual-port switchUp to protocol limit

Configuring two switches as Redundancy Managers in the same physical ring or failing to configure any Manager causes network failure or immediate broadcast storms.

Designating Ring Ports and Physical Wiring Rules

Ring redundancy requires strict physical discipline:
* Dedicated Ring Ports: Only specific ports configured as Ring Ports (typically Ports 1 and 2 or optical FO ports 7 and 8 on SCALANCE X-200 / XC-200) may participate in the ring.
* No Spurring on Ring Ports: End devices (servers, PCs, printers) must never be plugged into ring ports. End devices must connect to standard non-ring ports.
* Physical Ring Closure: Connect Ring Port 2 of Switch 1 to Ring Port 1 of Switch 2, continuing sequentially through all switches until Ring Port 2 of the final switch connects back to Ring Port 1 of Switch 1.

Step-by-Step Configuration in Web Based Management and HW Config

To configure an MRP ring using SCALANCE Web Based Management (WBM):

Connect a web browser to the SCALANCE switch IP address (e.g., 192.168.1.1).
Log in with administrative credentials.
In the navigation tree, expand Layer 2 -> Ring Redundancy.
In the Ring Redundancy configuration panel:
- Enable Ring Redundancy.
- Under Protocol, select MRP.
- Under Role, configure the primary switch as Manager, and all companion switches as Client.
- Assign Ring Port 1 (e.g., Port P1.1) and Ring Port 2 (e.g., Port P1.2).
Click Set Values to apply.
Harmonize in STEP 7 HW Config:
- In SIMATIC Manager HW Config, open properties for the PROFINET IO system -> Media Redundancy.
- Set the domain MRP role matching the WBM configuration (Manager or Client).
- Save, Compile, and download to the automation controllers.

Verification

To verify ring redundancy and measure failover performance:
1. In SCALANCE WBM, inspect the Ring Redundancy Status; verify that status displays Ring Closed and the Fault LED (F) is extinguished.
2. Initiate a continuous ping from an OS Server to a distributed I/O station: ping -t <Device_IP>.
3. Physically disconnect one Ethernet cable from Ring Port 1 of Switch 2; verify that the Redundancy Manager illuminates its red fault LED and status transitions to Ring Open.
4. Confirm that the continuous ping stream drops a maximum of 1 to 2 packets (< 200 ms failover) without dropping AS-OS or I/O communication.
5. Reconnect the cable; verify that the ring returns to Ring Closed seamlessly.

Limitations and Operational Precautions

Never connect an Ethernet cable to complete a physical ring before configuring the Redundancy Manager in software. Closing a physical Ethernet loop without an active Redundancy Manager causes an instantaneous broadcast loop that saturates switch backplanes and halts all plant communication within seconds. Always configure the Manager switch first, verify its settings, and only then complete physical ring wiring.

Engineer FAQ

Can SCALANCE switches run MRP and Spanning Tree (RSTP) simultaneously?

No. A switch port configured for MRP media redundancy cannot participate in Rapid Spanning Tree Protocol (RSTP). MRP and RSTP must be separated into distinct network domains.

What happens if two switches in the same ring are set as Redundancy Manager?

Both switches will attempt to block their secondary ring ports, segmenting the ring into isolated subnet islands and cutting off communication between devices.