Resolving SCALANCE XC206-2 and XB205-3 MRP Ring Error Reports

SCALANCE XC-200 and XB-200 managed switches are widely deployed in PROFINET ring topologies because they implement Media Redundancy Protocol (MRP) per IEC 62439-2 with sub-200 ms reconfiguration times. In a five-switch fiber ring, a recurring commissioning complaint is that surviving switches raise a "Module exists, Error" diagnostic in TIA Portal after one member is removed, powered down, or hot-swapped, even though the ring itself is forwarding traffic correctly. This article documents the root cause, the configuration that suppresses the diagnostic without breaking MRP, and the verification steps that prove the fix is in place.

1. Problem Description

When an MRP ring built with SCALANCE XC206-2 or XB205-3 switches loses one member (power removed, fiber disconnected, device swapped), the Media Redundancy Manager (MRM) opens its blocked backup port and the ring continues to forward traffic within the configured reconfiguration time. The functional behaviour of MRP is correct. The fault is purely diagnostic:

• One or more of the surviving MRC (MRP Client) switches report Diagnostic status: Module exists, Error in the TIA Portal device view.
• The error appears to be random: in any given 5-switch ring, typically 1-3 of the 4 clients will raise the diagnostic on a single member removal, and the switches that report vary event-to-event.
• No frames are lost beyond the standard MRP reconfiguration window (default 200 ms).
• Subsequent removals, additions, or re-insertions of ring members produce a new, different set of switches raising the diagnostic.

The fault is benign from a forwarding perspective but it floods the HMI/SCADA diagnostic stream with spurious errors and complicates the troubleshooting of real network issues. In safety-conscious plants, nuisance diagnostics can mask genuine link-loss events.

2. Affected Hardware and Firmware

Devices confirmed to exhibit the diagnostic fault, with their Siemens article numbers and confirmed firmware releases:

The same diagnostic propagation is implemented in the SINEC OS base image used by the SCALANCE XC-300, XR-300, and XM-400 families. Siemens revised the partner-port diagnostic in SINEC OS V2.4 (firmware V6.4.x for XC-200) so that the spurious partner-loss event is suppressed when "Any partner" is selected. Sites still running V4.x firmware must apply the configuration procedure in Section 8 to achieve the same effect.

3. MRP Protocol Fundamentals (IEC 62439-2)

Media Redundancy Protocol is defined by IEC 62439-2 and operates as a ring-recovery protocol with three roles:

• MRP Manager (MRM) - exactly one switch per ring. Sends test frames on both ring ports and observes their return. On detection of ring break, opens the blocked backup port and floods a topology-change frame.
• MRP Client (MRC) - all other ring members. Forwards MRP test frames transparently and reacts to reconfiguration commands from the MRM by flushing its MAC address table.
• MRP Auto-Manager (MRA) - optional role defined in IEC 62439-2 Edition 2; a switch that becomes an MRM only if no other MRM is present. SCALANCE implements this as the "Manager (Auto)" role.

Default MRP timing parameters on SCALANCE XC-200 / XB-200:

When the ring is healthy, the MRM blocks one of its two ring ports (the backup or secondary port) to prevent a Layer 2 loop. All other ring ports forward. If the MRM loses three consecutive test frames, it concludes the ring is broken, opens the blocked port, and floods a topology-change frame so all MRCs flush their MAC tables. The total reconfiguration time in the default configuration is approximately 200 ms, dominated by the MRM's test-frame loss detection plus the MAC flush on each MRC.

Reference for the official TIA Portal V20 ring-redundancy configuration walkthrough: Configuring ring redundancy for SCALANCE X in TIA Portal V20.

4. Root Cause: Ring Port Partner Detection

The SCALANCE firmware implements an additional link-state check on each ring port beyond the standard IEC 62439-2 test frame mechanism. The check looks for a defined partner port on the remote end of the ring link. When a switch is commissioned through TIA Portal topology discovery, the engineering station records which neighbour port is connected to each ring port and writes that binding back to the switch. The binding is stored as the "Ring port partner" attribute on each ring port.

If the partner switch disappears (powered off, removed, or both fiber strands cut to that switch), the surviving switch's ring port has physical link-down on at least one side. The firmware does not raise an MRP error in this case - the ring is simply degraded and the MRM handles the reconfiguration. However, the partner-binding state machine on the SCALANCE also reports the lost binding up to TIA Portal through the SNMP/PN-DCP interface, and the resulting diagnostic is the generic Module exists, Error string observed in the device view.

Why it appears random: the MRP state machine on each MRC updates independently. An MRC whose ring port(s) face the removed switch will see the link-down and report the partner mismatch immediately. An MRC on the opposite side of the ring only sees the topology-change flood from the MRM and may not generate the partner-mismatch event if the local link state is still up. The set of switches that report therefore depends on which switch was removed and on the port-to-port wiring of the ring at the moment of the event.

5. Why "Manager (Auto)" Fixes the Ring But Not the Diagnostic

Setting the MRP role to "Manager (Auto)" on every switch is the Siemens-recommended default for any ring where the user cannot guarantee a fixed manager. In "Manager (Auto)" mode, all switches are eligible to become the MRM. The MRP manager election algorithm (defined in IEC 62439-2 and refined for SINEC OS) selects the switch with the lowest MAC address to be the active manager. The ring will function regardless of which switch becomes the manager.

This setting fixes the "ring does not function" symptom because it eliminates the requirement to manually designate one MRM. It does not disable the partner-port detection that triggers the spurious TIA Portal diagnostic. The diagnostic is generated by the per-port partner state machine, not by the MRP role assignment, so the two issues are independent.

6. TIA Portal Topology Discovery vs. Real Network

When the project is commissioned in TIA Portal, the engineering station reads the actual topology from the devices using PROFINET Discovery and Configuration Protocol (PN-DCP). The discovered topology is stored in the project and used for three purposes:

1. Verify the as-built network matches the designed network.
2. Resolve device names and IP addresses for replacement devices.
3. Provide the link-status information that drives the Module exists diagnostic in the device view.

When a switch is physically removed, the TIA Portal topology view continues to expect the missing switch and raises an Error badge against the port whose neighbour disappeared. The SCALANCE may or may not forward that diagnostic upward, depending on the firmware version and on the partner-port configuration on the surviving switch. The relevant parameter on the SCALANCE is Ring port with partner - when set to Any partner, the SCALANCE does not bind the ring port to a specific neighbour device and therefore does not generate a partner-loss diagnostic. When set to a specific partner (the default after TIA Portal topology commissioning), the partner-loss event is reported.

7. Configuration Parameters Reference

WBM and TIA Portal expose the following ring-related parameters on the SCALANCE XC-200 / XB-200 line. The parameter names match the WBM (Web-Based Management) interface and the TIA Portal device configuration under Layer 2 > Ring redundancy.

The two parameters that control the spurious diagnostic are MRP role and Ring port N partner. The Manager (Auto) setting fixes the ring function; the Any partner setting fixes the diagnostic.

8. Step-by-Step Resolution Procedure

The procedure below eliminates the spurious Module exists, Error diagnostic while preserving correct MRP ring operation. The procedure must be performed on every switch in the ring, because each switch has its own partner-port binding state.

8.1 Prerequisites

• TIA Portal V17 or later (the official Siemens documentation referenced uses V20: TIA Portal V20 SCALANCE ring redundancy).
• SCALANCE firmware V4.3 or later on each switch (verify via WBM: Information > Versions).
• Administrative access to the WBM of each switch (HTTPS, default user admin, default password admin).
• Read/write access to the TIA Portal project that contains the SCALANCE devices.
• Backed-up configuration: WBM > Information > Backup > Save to local PC, or use the backup CLI command from the SSH console.

8.2 Configure MRP Role on All Switches

1. Open TIA Portal and the device configuration for each SCALANCE switch in the ring.
2. Navigate to Layer 2 > Ring redundancy.
3. Set the MRP role to Manager (Auto) on every switch in the ring. This allows any switch to take over as manager if the configured manager is lost.
4. Set Ring port 1 and Ring port 2 to the actual physical ring ports on each switch. On the XC206-2 these are the SFP/fiber ports (P1 and P2 by default). On the XB205-3 these are also the SFP/fiber ports (P1 and P2 by default).
5. Compile the TIA Portal project and download the configuration to all switches. Use "Go online > Download to device" so the running config is updated and persisted to the startup config.

8.3 Disable Specific Partner Binding

1. For each switch, in the same Layer 2 > Ring redundancy panel, locate the Ring port 1 partner and Ring port 2 partner fields.
2. Set both to Any partner. The SCALANCE will accept any MRP-capable device on the ring port and will suppress the partner-mismatch diagnostic when a neighbour switch is replaced.
3. Save the configuration to the device (the WBM equivalent: System > Save > Save to startup config).

8.4 Optional: Verify with WBM

1. Open the WBM of each switch at https://<switch-ip>.
2. Navigate to Layer 2 > Ring Redundancy. Confirm the role shows Manager (Auto) and both ring ports show Any in the partner column.
3. Navigate to Information > Diagnostics Buffer and clear the buffer to baseline future diagnostics.
4. Verify the ring status: Information > Redundancy should show Ring is closed and identify exactly one manager (lowest MAC address in the ring).

8.5 Recompile Topology in TIA Portal

1. In the TIA Portal project tree, right-click each SCALANCE device and select Go online.
2. Open Online & Diagnostics > Topology for each switch.
3. Click Compare offline/online to verify the project topology matches the physical ring.
4. If TIA Portal still raises the partner diagnostic on a switch you have already reconfigured, run Discover from Project tree > Devices & Networks to refresh the topology view.

9. Verification Checklist

After completing the procedure, perform the following checks to confirm the fault is resolved. All checks should pass within 30 seconds of the change.

1. Confirm ring operation: WBM > Information > Redundancy > MRP shows Ring is closed and identifies one manager (lowest MAC address in the ring, marked with (M)).
2. Confirm diagnostic suppression: power down one of the client switches. After 30 seconds, no Module exists, Error should appear on any of the remaining switches in TIA Portal.
3. Confirm reconfiguration time: ping a device on the ring across the failed segment. The maximum observed loss should be approximately 200 ms (the MRP default reconfiguration time) - measure with a continuous ping at 100 ms interval and count the gap.
4. Confirm test frame rate: in the WBM, Information > Redundancy > MRP shows approximately 50 test frames/sec on a healthy ring (interval = 20 ms).
5. Confirm MAC flush: during the reconfiguration, the WBM Information > Statistics > MAC Address Table should show entries being relearned after the topology-change flood.
6. Re-add the removed switch: power it back up, wait 30 seconds, confirm the ring closes and no new diagnostic appears.
7. Re-test with a hot swap: swap a ring member with a spare SCALANCE of the same model. Confirm the spare is auto-named by PN-DCP and no spurious diagnostic fires.

10. Best Practices and Third-Party Compatibility

Use these guidelines when designing or commissioning a SCALANCE MRP ring.

• Use Manager (Auto) on all switches if the design does not mandate a fixed manager. The MRP manager election algorithm takes only ~200 ms to converge and the ring is functional throughout the election.
• If a fixed manager is required (for example, to pin the manager to a known switch for traceroute purposes), hardcode the MRM on a switch with a stable, low MAC address. Document this in the network drawing.
• Set ring port partners to Any partner on all MRCs to allow hot-swap of any ring member without spurious diagnostics. Only the MRM should keep "Specific partner" bindings to validate ring closure.
• Use distinct VLANs for the ring management traffic and the user traffic to limit the broadcast domain size and prevent the MRP test frames from leaking into the user VLAN.
• Always commission the SCALANCE ring through TIA Portal topology discovery (Online > Devices & Networks > Topology view > Go online) to ensure the project accurately reflects the as-built network.
• Do not mix HRP (High-availability Redundancy Protocol) and MRP in the same ring. HRP is supported only on the SCALANCE X-300 / XM-400 lines and is not interoperable with MRP.
• For PROFINET conformance, ensure the MRP test frame interval is at least 20 ms on rings carrying PROFINET IRT traffic, as defined in the PROFINET installation guidelines.
• Interoperability with third-party MRP devices (Hirschmann, Phoenix Contact, Stratix, Westermo) is supported per IEC 62439-2. The SCALANCE may be configured as MRM (Manager) and the third-party device as MRC (Client), or vice versa. Verify the third-party device supports the Any partner / Watchdog partner mode if you need to suppress the partner diagnostic on the SCALANCE side.

11. Troubleshooting Matrix

Use this matrix to triage the most common MRP-related complaints on SCALANCE XC-200 / XB-200 rings.

Frequently Asked Questions