Six sets of OptiX OSN3500 that are configured with the N2SL64 boards form a 10 Gbit/s MSP ring. During the MSP ring switching test, the fiber is cut. The services, however, are not interrupted and the MSP switching is normal. Reconnect the fiber. The switching is restored after 10 minutes. The services, however, are interrupted immediately and are automatically restored in four minutes. At the same time, the optical board where switching is performed reports the B3_SD alarm. The B3_SD alarm lasts for two hours and then is automatically cleared. Check on the T2000. It is found that the MSP switching is normal. Replace the cross-connect board. It is found that the fault persists.