Para lograr la función de FEC, connect the two pairs of ports of the two switches with two lines at the same time, pero descubre que cada conmutador es siempre un puerto normal, y el otro puerto muestra una luz roja. ¿Es culpa del puerto del switch??

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.