How interface state, counters, cable condition, transceivers, and link negotiation identify physical network faults.
Physical-layer problems can be total failures or intermittent degradations. A link indicator only shows that some electrical or optical signal is present; it does not prove that frames are being transmitted cleanly.
| Evidence | Likely area to inspect first |
|---|---|
| Interface is down/down | Cable seating, power, port state, transceiver presence, or failed hardware |
| CRC, frame, or input errors increase | Cable, connector, termination, interference, optics, or signal quality |
| Link flaps after movement | Patch lead, wall jack, connector, or physical strain |
| Late collisions or incompatible speed/duplex settings | Negotiation or forced interface mismatch on legacy links |
| Fiber link does not establish after an optics change | Module compatibility, fiber type, wavelength, polarity, or cleanliness |
CRC errors indicate frames that fail a cyclic redundancy check. Their meaning depends on platform counters, but a rising pattern on one interface is strong evidence to inspect the local physical path before changing routing or DNS. A duplex mismatch occurs when link ends use incompatible duplex settings and can cause poor throughput, errors, and collisions on technologies that support half duplex.
1Gi1/0/12 is up, line protocol is up
2input errors 57, CRC 42, frame 9
3output errors 0
The link is operational but not healthy. Check the cabling and termination, then the connected port and optics, while comparing the counters before and after each controlled test.
For facilities factors such as power, cooling, electromagnetic interference, and cable management, see Network Installations, Power, and Environment.