An optical network unit, ONU, wherein the ONU comprises a first Maintenance association End Point and a second Maintenance association End Point, and the ONU further comprises: a monitoring unit, configured to transmit a monitoring packet through the first Maintenance association End Point on a first link connected to an aggregation side switch device, so as to monitor the first link, wherein the first link comprises a first sub-link between the ONUand a first optical line terminal, OLT, and a second sub-link between the first OLT and the aggregation side switch device, and the first Maintenance association End Point is a port corresponding to the first link; a switching unit, configured to perform switching from the first sub-link of the first link to a first sub-link of a second link if it is detected that a link fault occurs on the first link, wherein the second link is a link, except the first link, between the ONU and the aggregation side switch device, and the first sub-link of the second link is a link between the ONU and a second OLT; and a sending unit, configured to send a notification message to the secondOLT, wherein the notification message is used to notify the second OLT of occurrence of link switching and a cause for the switching, so that the second OLT enables a transmission port, wherein the sending unit is further configured to send an automatic protection switching message to the aggregation side switch device through the second Maintenance association End Point on the second link, wherein the automatic protection switching message is used to notify the aggregation side switch device of the occurrence of the link switching, so that the aggregation side switch device performs switching from the second sub-link of the first link to a second sub-link of the second link after receiving the automatic protection switching message, wherein the second sub-link of the second link is a link between the aggregation side switch device and the second OLT, and the second Maintenance association End Point is a port corresponding to the second link. a monitoring unit, configured to transmit a monitoring packet through the first Maintenance association End Point on a first link connected to an aggregation side switch device, so as to monitor the first link, wherein the first link comprises a first sub-link between the ONUand a first optical line terminal, OLT, and a second sub-link between the first OLT and the aggregation side switch device, and the first Maintenance association End Point is a port corresponding to the first link; a switching unit, configured to perform switching from the first sub-link of the first link to a first sub-link of a second link if it is detected that a link fault occurs on the first link, wherein the second link is a link, except the first link, between the ONU and the aggregation side switch device, and the first sub-link of the second link is a link between the ONU and a second OLT; and a sending unit, configured to send a notification message to the secondOLT, wherein the notification message is used to notify the second OLT of occurrence of link switching and a cause for the switching, so that the second OLT enables a transmission port, wherein the sending unit is further configured to send an automatic protection switching message to the aggregation side switch device through the second Maintenance association End Point on the second link, wherein the automatic protection switching message is used to notify the aggregation side switch device of the occurrence of the link switching, so that the aggregation side switch device performs switching from the second sub-link of the first link to a second sub-link of the second link after receiving the automatic protection switching message, wherein the second sub-link of the second link is a link between the aggregation side switch device and the second OLT, and the second Maintenance association End Point is a port corresponding to the second link.