Network Troubleshooting · Topic 2 of 6
Cabling and Physical Interface Issues
Troubleshoot wrong cable types, crosstalk, attenuation, bad terminations, interface error counters, port states, duplex mismatch, PoE and transceivers.
- 5 min
- No link light points to the cable, the port or the far end first.
- Attenuation grows with length. Copper Ethernet is limited to 100 meters.
- CRC errors suggest damaged frames from cable, interference or duplex mismatch.
What it is
Many network faults begin at layer 1 or layer 2: a bad cable, a worn connector, a mismatched transceiver or a port in the wrong state. These problems are common, so the exam tests them heavily.
The skill is to read symptoms, such as a dead link light or rising error counters, and decide whether the cause is the cable, the interface or the settings.
How it works
Cable problems
| Problem | Meaning and typical symptom |
|---|---|
| Incorrect cable type | Wrong category for the speed, shielded versus unshielded when shielding is needed, or single mode fiber where multimode is required |
| Crosstalk | Signal from one wire pair leaks into another, causing errors |
| Interference | Outside electrical noise, from motors or lights, corrupts the signal (electromagnetic interference, EMI) |
| Attenuation | The signal weakens over distance, so runs that exceed the limit, such as 100 meters on copper, fail or drop |
| Improper termination | Wires crimped in the wrong order, loose or damaged connectors |
| Transmit (TX) and receive (RX) transposed | The TX and RX pairs are reversed, so the link does not come up |
A cable that is too long, kinked, run beside power cables or poorly terminated usually gives intermittent faults rather than none at all. Fiber needs clean connectors, and a dirty or damaged end face causes signal loss.
Interface counters
Interface statistics reveal problems that a link light cannot show.
| Counter | Likely meaning |
|---|---|
| Cyclic redundancy check (CRC) errors | Frames damaged in transit, often from a bad cable, interference or duplex mismatch |
| Runts | Frames smaller than the minimum size, often from collisions or a faulty device |
| Giants | Frames larger than allowed, possibly from a misconfigured maximum transmission unit (MTU) or a faulty device |
| Encapsulation errors | The two ends disagree on the frame format, for example different trunk tagging or encapsulation settings |
| Drops | Frames discarded, often because of congestion or full queues |
Clear the counters, wait and check again. A counter that keeps rising shows an active problem, while an old count may be history.
Port status
| Status | Meaning | Typical fix |
|---|---|---|
| Up | Link is active | None |
| Down | No link detected, physical problem | Check cable, connector and the far end |
| Administratively down | An administrator shut the port | Enable the port if it should be in use |
| Error disabled | The switch shut the port after detecting a problem, such as a port security violation | Fix the cause, then re-enable the port |
| Suspended | The port has been removed from use because of a mismatch, for example in a bundle | Correct the configuration |
Speed and duplex mismatch
If one end is set to full duplex and the other end to half duplex, the link comes up but performs badly and shows collisions and errors. The usual fix is to set both ends to auto negotiate, or to match them manually.
Power over Ethernet problems
If a Power over Ethernet (PoE) device will not power on, check whether the switch has exceeded its power budget, whether the port supports the standard the device needs, and whether the cable is good. A device that needs more power than the standard supplies will not start.
Interface hardware and transceiver problems
A failed network interface card (NIC) or switch port gives a dead link even with a good cable and matching settings. Swap the cable, the port or the NIC in turn to isolate which part is bad.
| Issue | Result |
|---|---|
| Wrong transceiver type or wavelength | No link or very weak signal |
| TX and RX fibers swapped | No link until the pair is reversed |
| Cable and transceiver mismatch | For example, a single mode module on multimode fiber, which causes errors or no link |
| Weak signal strength | Dirty connectors, long run or damaged fiber |
Both ends of a fiber link must use compatible modules, and the fiber must match them.
What to remember for the exam
- No link light points to the cable, the port or the far end first.
- Attenuation grows with length. Copper Ethernet is limited to 100 meters.
- CRC errors suggest damaged frames from cable, interference or duplex mismatch.
- Runts are too small, giants are too large and drops are discarded frames.
- Error disabled means the switch shut the port after a violation. Administratively down means someone shut it.
- A duplex mismatch causes slow links with errors, but the link stays up.
- Check the PoE power budget and standard when a powered device will not start.
- Swapped TX and RX or mismatched transceivers give no link on fiber.
When a link works but is slow, look at the counters before replacing the cable. Rising CRC errors with a duplex mismatch is a classic answer.
Common mistakes
Replacing the cable first for every fault. Check port status and settings, which are quicker to confirm.
Reading a lit link light as proof of health. A link can be up and still have a duplex mismatch or errors.
Confusing administratively down with down. One is a deliberate shutdown, and the other means no signal.
Mixing fiber types. Single mode and multimode equipment and cable must match.
Forgetting the far end. Many faults are at the other end of the cable.
Do not look into the end of a fiber or a transceiver port. The invisible laser light can damage your eyes.
Check yourself
Try these three.
A port is "error disabled". What does that mean?
The switch shut the port after detecting a problem such as a security violation, and the cause must be fixed before it is re-enabled.
What does a rising CRC error count usually indicate?
Frames are being damaged, often by a bad cable, interference or a duplex mismatch.
A PoE phone does not power on although the cable is good. What should you check?
The switch's PoE power budget and whether the port supports the standard the phone needs.
Topic 2 of 6 in Network Troubleshooting
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