In the network shown in the following figure, NE6 is the management node, and services are configured between NE2 and NE6. In addition, the tributary board of NE2 is not in position.
Under normal conditions, the service flow, indicated by the red real line, is as shown in the figure. When NE6 starts a westward forced switching, NE2 switches from the IDLE page to the eastward switching page. The service flow, indicated by the blue dashed line, is as shown in the figure.
In the process of the switching, if the tributary board of NE2 is in position, the OTN signal on the protection link is stable and there are no abnormal alarms on the protection channel. But if the tributary board of NE2 is not in position, the OTN signal on the protection link needs to be regenerated by an NS2 board (according to certain Recommendations). The regeneration operation requires a certain period of time. The protection channel may detect abnormal alarms before the OTN signal regeneration is completed and immediately reports an SF alarm, which results in the failure of the forced switching.