To achieve the function of FEC, connect the two pairs of ports of the two switches with two lines at the same time, but find that each switch is always a normal port, and the other port shows a red light. Is it the fault of the switch’s port?

In general, this situation switch is normal, because the two switches are connected by two ports, so the switch thinks that there is a loop, it automatically breaks one of them, and the corresponding port is Down (the port showing the red light). The solution is to turn on the Spanning tree function (the default is on) to let the switch know that the two ports are FEC functions and are logically one port.