Port loopback testing can detect whether the status of a port is normal, and is often used to locate port faults in network maintenance work. Port loopback testing can be divided into local loopback testing and remote loopback testing. With the increase of Ethernet speed, the complexity of the PHY layer has greatly increased. Starting from 10G Ethernet, multiple sub layers of the PHY layer began to appear in the form of MMD devices, and the MAC chip also integrated internal PHY functions. The schematic diagram in the following text also adopts this form. In local loopback testing, ports are sent out from the MAC layer to the PHY layer. Local loopback testing is relatively common, usually divided into MAC inner loop, PHY inner loop, and port outer loop. These loopback testing methods are simple and easy to use, and Ethernet devices generally support them. 1. The testing scope of MAC inner ring is limited to MAC chips. The scope of PHY inner loop testing covers the PCS sublayer from MAC chip to PHY chip. 3. The outer ring outer ring test requires constructing a physical loop outside the port. The electrical port uses a self ring head, and the optical port can be connected to the TX and RX ends of the optical module using a fiber optic