It looks like a LAN cable, tests like a LAN cable, and runs a laptop perfectly. But inside, it only has half the wires — and a gigabit uplink is exactly where that stops being forgivable.
Here is the trap in one line: 100 Mbps uses two pairs; gigabit uses four. A cable that wires only the two pairs Fast Ethernet needs will run a PC so convincingly that nobody suspects it. Then someone uses it — or it was always used — as the link between two switches, and the uplink either refuses gigabit, falls back to 100, or flaps under load. Before you blame the switches in that situation, pick up the cable and count what is actually inside the plug.
Nobody sets out to buy a bad cable — they arrive quietly. Two-pair cables ship free in the box with some routers and CCTV kits, get sold as slim "flat" travel cables, and come attached to cheap splitters. Some are really telephone-style cords with RJ45 plugs. They look identical to a full cable from the outside; the missing pairs are invisible unless you look through the plug and count the copper, or flex the cable and feel how thin it is. Phone charging combos and "LAN + phone" economy cables have the same skeleton. They were built down to a price for a 100 Mbps world.
Fast Ethernet at 100 Mbps transmits on pins 1 and 2 and receives on pins 3 and 6 — exactly the two pairs this cable has. The laptop negotiates 100 Mbps, browses, prints and copies small files without a single complaint. Every casual test passes. The cable is not "fine", though — it is capped, and the cap only shows when something asks for the other two pairs.
Gigabit Ethernet (1000BASE-T) is a different animal: all four pairs carry data simultaneously in both directions, with echo cancellation separating the directions. With pairs three and four physically absent, there is nothing to negotiate with. Depending on the switches, the uplink will refuse to link, drop to 100 Mbps, or — worst of all — link at gigabit for a while and fall over under load as the ports keep retraining. Any of the three reads as "the switch is faulty" until someone counts the wires in the plug.
Honesty cuts both ways: a two-pair cable is not dangerous, and for a single 100 Mbps device — an old printer, a basic IP phone — it works within its design. It has no business as an uplink, where every user on the far switch shares its ceiling, nor anywhere gigabit was planned. Pull it out of uplink duty, label it clearly if it stays in the building at all, and put a certified full-pair patch cord in its place. The price difference is a few rupees; the troubleshooting difference, as the video shows, is an afternoon.
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 VideoBecause for Fast Ethernet it genuinely works — pins 1, 2, 3 and 6 are all 100 Mbps uses. Such cables ship bundled with routers and kits, or are sold as slim and flat economy cords. They are not fake, just built for a 100 Mbps job. They fail only when asked for gigabit, which needs all four pairs at once.
No. It will not harm the switch or the PC — it simply cannot carry gigabit. The ports will fall back to 100 Mbps, refuse the link, or retrain repeatedly depending on the models. The damage is to your time and your network speed, not to the hardware.
Yes — for a single end device that only needs 100 Mbps, such as an older printer or a basic IP phone, it works within its limits. Label it so nobody promotes it later. Never use it as a switch-to-switch uplink, where the whole far side of the network shares its 100 Mbps ceiling.