在以太网无源光网络/吉比特无源光网络(EPON/GPON)系统中，光网络单元(ONU)用户端口成环将会造成同一个无源光网络(PON)口下所有的业务中断。文章首先分析了EPON/GPON系统中造成ONU用户端口成环的常见原因以及成环后所带来的严重后果，然后列出了5种可以破除用户侧端口环路的方法，并在此基础上提出了采用“特殊报文探测法”破除用户环路。利用特殊报文的收发和比对可以自动地发现和阻止用户端口成环。

In EPON/GPON systems, ONU user ports loopbacks will cause the discontinuation of all the services at the same PON user ports. This paper first analyzes the common causes for the loopbacks and the serious consequences arising from them. Then, it lists five methods that can be used to prevent the ONU’s UNI ports from loopbacks and, on this basis, presents a “special message detection” method, which can be used to automatically discover and block the user port loopbacks by sending, receiving and comparing the special messages.