Sometimes you need to prove that an Ethernet port you want to use is in good working condition, but maybe you’re having problems confirming continuity across the port to the next piece of equipment. A simple solution is to create a Loopback (LPBK) Plug and plug it into the port in question. If the port is in good working condition, then the LNK LED light will light-up to indicate that this port sees the next device and everything is good.
Well, in reality, you merely held-up a mirror for your device to see itself. As long as it was satisfied that connectivity was achieved, then the port itself is in great condition.