Cable Diagnostics (TDR)

What is TDR Cable Diagnostics?

Many Catalyst switches include a built-in Time Domain Reflectometer (TDR) in their DSP/PHY. The test cable-diagnostics tdr command sends an electrical pulse down each of the four copper pairs on an RJ-45 port and measures the reflection to determine pair status and, when there's a fault or a remote link partner, the approximate distance to it — all without unplugging a single cable.

How It Works

1. The PHY injects a test pulse onto each pair (A, B, C, D).
2. The pulse travels down the cable and reflects back when it hits an impedance change — an open end, a short, a bad connector, or a properly terminated link partner.
3. The switch measures the round-trip time of the reflection and converts it to a distance using the cable's nominal velocity of propagation.
4. Results are reported per pair: status (Open / Short / Terminated / Pair up) and distance to the fault or termination point.

Requirements & Limitations

• Copper RJ-45 ports only — not supported on SFP/fiber interfaces.
• Running the test on an active link briefly interrupts traffic (link flap of roughly 1–2 seconds) — schedule it during a maintenance window on production ports.
• Distance readings are estimates, typically accurate to ± 10 meters, and become unreliable on very short cable runs (under ~15 m / 50 ft).
• Not all platforms/linecards support it — older fixed-config switches and most router WAN ports do not implement TDR.
• The interface should be administratively up (no shutdown) before testing; an admin-down port typically returns Not Completed.

Platform Support

Fiber Doesn't Use TDR

TDR relies on electrical reflections and only works on copper pairs. For fiber links, the equivalent diagnostic is Digital Optical Monitoring (DOM), built into most SFP/SFP+ transceivers — it reports real-time optical power, temperature, voltage, and bias current.

Optical Diagnostic Commands

Typical DOM Thresholds

Exact thresholds vary by transceiver model — always check show interfaces transceiver detail for the specific high/low alarm and warning values programmed into that SFP.

Copper Cable Fault Checklist

1. Check the obvious first — reseat both ends of the cable and confirm it's fully clicked into the jack before running TDR.
2. Confirm the interface is up — show interfaces <intf> status. An admin-down port will report Not Completed.
3. Run the test — test cable-diagnostics tdr interface <intf>, then wait a few seconds.
4. Read the results — show cable-diagnostics tdr interface <intf>, or paste them into the Result Interpreter tab above.
5. Open/Short at a short distance (a few meters) usually means damage near the connector — check for a bad crimp or a damaged jack.
6. Open/Short at a longer distance points to physical damage mid-run — a staple through the cable, a chewed section, or a kink/crush point.
7. Only some pairs bad? Only 2 of the 4 pairs are used at 100 Mbps (Fast Ethernet); a fault on an unused pair won't affect a 100 Mb link but will block Gigabit, which needs all four.
8. Check pinout compliance — mixed T568A/T568B ends on the same run cause split pairs. See the Ethernet Cable Standards tool for pinout reference.
9. Verify against cable category limits — runs beyond 100 m (328 ft) for copper Ethernet will show marginal or failing links regardless of TDR results.
10. Still inconclusive? — swap in a known-good patch cable to isolate whether the fault is in the fixed cable run or a patch cord.

Common Errors