Check CRC errors first
A flaky cable is the most common cause of ring oscillation. Check CRC error counters on ring ports.
11.4 MRP Troubleshooting
The previous chapter covered MRP timing parameters and how to tune them for faster recovery. When the ring does not behave as expected, systematic troubleshooting identifies the root cause. MRP problems fall into a small number of categories.
Why a Systematic Approach Matters
Section titled “Why a Systematic Approach Matters”MRP ring problems share similar symptoms: devices become unreachable, the MRM reports the ring as open, or topology changes appear in the log. Without a structured approach, engineers chase symptoms instead of causes. The four most common root causes are: wrong VLAN, wrong Domain UUID, two MRMs, or a flaky cable.
Symptom: Ring is Open (No Physical Failure)
Section titled “Symptom: Ring is Open (No Physical Failure)”The MRM reports the ring as open even though all cables are connected.
| Cause | Diagnosis | Fix |
|---|---|---|
| MRP not enabled on all switches | Check MRP status on each switch | Enable MRP on all ring switches |
| Wrong Domain UUID | Compare UUID across all switches | Set the same UUID on all switches |
| MRP VLAN mismatch | Check MRP VLAN on each ring port | Set the same MRP VLAN on all ring ports |
| Ring port not in MRP VLAN | Check VLAN membership of ring ports | Add ring ports to MRP VLAN |
| Disabled ring port | Check port status | Enable the port |
Symptom: Ring Oscillates
Section titled “Symptom: Ring Oscillates”The ring continuously changes state. Topology change events appear in the log every few seconds.
Symptom: Failover Takes Too Long
Section titled “Symptom: Failover Takes Too Long”The ring recovers, but recovery takes longer than 200 ms.
| Cause | Fix |
|---|---|
| MRP_LinkDown disabled | Enable MRP_LinkDown |
| Test interval too long | Reduce test interval to 10 ms |
| Switch CPU overloaded | Reduce other traffic or upgrade switch |
Symptom: Traffic Lost After Failover
Section titled “Symptom: Traffic Lost After Failover”The ring recovers (MRM shows ring closed) but some devices are unreachable.
| Cause | Fix |
|---|---|
| Stale ARP entries | Clear ARP cache on affected devices |
| MAC table not flushed | Verify MRP_TopologyChange propagation |
| VLAN missing from new path | Add missing VLAN to all ring ports |
Diagnostic Tools on Hirschmann HiOS
Section titled “Diagnostic Tools on Hirschmann HiOS”| Tool | Location | Use |
|---|---|---|
| MRP Status | Switching → L2-Redundancy → MRP | Ring state, port roles, domain |
| Event Log | Diagnostics → System Log | Topology change events |
| Port Statistics | Basic Settings → Port Statistics | CRC errors, link up/down counts |
| Port Mirroring | Diagnostics → Port Mirroring | Capture MRP frames with Wireshark |
To capture MRP frames with Wireshark, configure port mirroring on the switch to mirror a ring port to a monitor port. Apply the Wireshark filter mrp or eth.type == 0x88e3. If no MRP_Test frames appear, MRP is not running on the MRM or the ring port is not in the correct VLAN.
Key Takeaways
Section titled “Key Takeaways”Four causes cover most problems
Wrong VLAN, wrong Domain UUID, two MRMs, or a flaky cable. Check these four first.
What Comes Next
Section titled “What Comes Next”MRP runs on Hirschmann switches. The next chapter introduces the Hirschmann product families, the HiOS operating system, and configuration best practices, providing the practical knowledge needed to deploy and manage these switches in production.
References
Section titled “References”- IEC 62439-2:2016 — Media Redundancy Protocol
- Hirschmann. (2023). User Manual: HiOS MRP Diagnostics. Belden/Hirschmann.
- Wireshark Foundation. (2024). Wireshark User’s Guide. https://www.wireshark.org/docs/wsug_html/