A method and device for detecting a loopback link, as well as an optical line terminal (OLT) and an optical network terminal (ONT). The invention solves the problem of the inability to determine with precision the ONT experiencing a loopback link, which can cause the user excessive service disconnections. The method comprises: an OLT sends a loopback detection command to ONTs; obtains loopback detection messages returned by the ONTs in accordance with the loopback detection command, said messages comprising the positions of source ONTs issuing the loopback detection messages, source port information, the positions of target ONTs receiving loopback detection messages, and target port information; and determines the ONT experiencing a loopback link on the basis of the positions of the source ONTs, the source port information, the positions of the target ONTs, and the target port information. By means of OLT and ONT cooperation, the technical solution allows for the precision detection of the ONT port on which a loopback link is occurring and, on the basis of ONT and ONT-port priority rankings, allows for the closure of service corresponding to VLAN information on an ONT port, thereby avoiding excessive service disconnections for the user and enhancing the user experience.
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