Perform an external loopback test to check whether the optical module is normal. By looping the transmitted signal (Tx) directly back to the receiving end (Rx), it enables a closed test without requiring a live network connection. What is a loopback test? The loopback test is a common testing. When troubleshooting a suspect port or verifying new hardware, a fiber-optic loopback test gives you a fast, definitive answer on whether an interface is healthy. The methodology is simple: start at the physical layer and work your way up the stack, confirming each layer before moving to the next. For more background theory, please read on.