The port LED is the first witness in any uplink fault — but only if you read it properly. No light, light at the wrong speed, and light with failing traffic are three different confessions pointing at three different repairs.
Ask a room of technicians what a link light means and most will say "connected". That one word hides three completely different situations, and each sends you to a different repair. No light means the two ports never agreed to talk — a physical break or a dead port. Light at 100 Mbps on gigabit hardware means pairs are missing. Light at full speed with failing traffic means the signal itself is being damaged. This guide teaches you to read the lights the way the video's troubleshooting does — as evidence, not decoration.
A completely dark LED means no electrical link was ever established. Work the physical list first, in this order:
Notice what is not on this list: IP addresses, VLANs and drivers. A dark LED is below all of that — the conversation never started, so there is nothing to configure yet.
When gigabit ports link at 100 Mbps, they are telling you only two pairs are usable. Gigabit needs all four pairs; when one pair is open, split or badly damaged, many ports gracefully fall back to a two-pair speed instead of refusing outright. Suspects, in order: a two-pair cable (some cheap or bundled cables wire only pins 1, 2, 3 and 6), one damaged pair in a long run, or a crimp where a single wire is not making contact. This is also the state that fools people — "the link works" — while every user on the far switch shares a 100 Mbps straw.
The nastiest state: full-speed lights, yet pings drop, shared folders crawl and calls break up. The link negotiated, but the signal is being damaged — classic split pairs, too much untwist at the plugs, a run squeezed alongside power cables, or a duplex mismatch. Confirm it instead of guessing:
ping -t to the far switch's gateway address and watch for replies that vanish in bursts.An activity LED blinking only means something is moving — including retransmissions of packets that failed the first time. A busy-looking uplink can be re-sending a third of its traffic. Judge a link by its negotiated speed plus its error counters plus a real file copy, never by how enthusiastically the light flickers. Conversely, a steady, calm LED at 1000 Mbps with zero errors is what "healthy" actually looks like.
The discipline that separates diagnosis from wandering: record the starting state — LED on or off, negotiated speed, error counter value — then change one thing (cable, port, or setting) and record the new state. Three changes in, you will know which single change moved the needle. Technicians who swap cable, port and settings in one motion fix the fault sometimes — and learn nothing every time, which is why the same fault eats another hour next month.
See the live troubleshooting in the video — the same cable tested with a laptop, then between two switches, and the wiring fault found on camera.
▶️ Watch Full VideoStart by checking the negotiated speed and the port error counters. Full speed with climbing CRC errors means a damaged signal — split pairs, a crushed run, or interference. Full speed with zero errors means the physical link is fine and the fault sits higher up, in VLAN or IP configuration. The LED alone cannot separate those two; the counters can.
On gigabit switches, yes — it means the ports could only agree on two usable pairs out of four. A two-pair cable, a damaged pair, or one bad crimp is the usual cause, and every device behind that uplink now shares 100 Mbps. It will look 'working' in casual use and collapse under real load.
Test, do not guess. Join the switches with a short factory patch cord. Link light appears: the original cable or its fixed run is open somewhere — cut, crushed, or badly crimped. Still dark: move the cord to different ports, one switch at a time. A port that never lights with any cable or any device is the dead one.