How to Test and Troubleshoot LAN Cables on Cisco Switches Using TDR
Introduction
LAN cables are the foundation of any wired network. Even a small issue in a cable can lead to problems like frequent disconnections, slow network speed, or complete loss of connectivity.
Cisco switches include a built-in diagnostic feature called Time-Domain Reflectometer (TDR). This tool allows you to test and troubleshoot Ethernet cables directly from the switch itself, without using any external cable tester.
What is TDR on Cisco Switches?
TDR is a cable testing technology that sends signals through a copper Ethernet cable and analyzes how those signals are reflected back. Based on this reflection, the switch can identify common cable faults.
TDR helps detect:
- Open circuits (broken or unplugged cables)
- Short circuits (wires touching each other)
- Cable length
- Distance to the fault location
- Pair or impedance mismatches
This feature works only on copper Ethernet cables such as Cat5e, Cat6, and Cat6a.
Check if Your Switch Supports TDR
Not all Cisco switch models support TDR on every port. You can check TDR support using the command below:
show controllers ethernet-controller phy | include TDR
If TDR is supported, you will see output confirming its availability for the interface.
How to Run a Cable Test
To run a TDR test on a specific interface, use the following command:
test cable-diagnostics tdr interface Gi1/0/5
If your switch supports testing all interfaces at once, you can run:
test cable-diagnostics tdr all
The test usually takes a few seconds to complete.
Viewing Cable Test Results
After the test is complete, view the results using:
show cable-diagnostics tdr interface Gi1/0/5
The output typically shows:
- Cable status (Normal, Open, Short, etc.)
- Length of each wire pair
- Exact distance to the fault (if any)
Understanding the Test Results
| Status | Description |
|---|---|
| Normal | The cable is healthy and working properly |
| Open | The cable is broken or disconnected |
| Short | Two or more wires are touching |
| Impedance | Mismatch in twisted pairs or poor termination |
| Unknown | No cable detected or feature not supported |
Best Practices for LAN Cable Testing
- Always label cables and switch ports clearly.
- Avoid sharp bends and tight cable bundles.
- Test critical links regularly to prevent unexpected failures.
- Replace damaged cables immediately.
- For very long cable runs or high-speed links, consider using an external cable tester.
Important Notes
TDR works only on copper Ethernet cables. Fiber optic cables require optical testing tools.
The interface must be administratively up. TDR tests do not work on shutdown ports.
Regular cable testing helps avoid hidden network problems and improves overall reliability.
Conclusion
Cisco’s TDR feature is a powerful and convenient tool for diagnosing LAN cable problems without additional hardware. It allows network administrators to quickly identify cable faults, locate problem areas, and reduce troubleshooting time.
By using TDR regularly and following proper cabling best practices, you can improve network stability, reduce downtime, and ensure consistent performance across your wired network.