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Figure 1 shows an SNCP over MSP ring.
NE1, NE2, NE3, NE4, and NE5 form an MSP ring, with an STM-1 chain An SNCP service is created between NE1 and NE3. The working source of this VC-4 service is on the working path of the MSP protection group and the protection source is on the STM-1 link. The SNCP hold-off time is 100 ms. When a fiber cut occurs for the first time (between NE1 and NE2), the MSP switching is normal. When a fiber cut occurs for the second time occurs (between NE3 and NE4), the SNCP service is interrupted but the SNCP switching does not occur.
Set the HP_UNEQ alarm as a trigger condition of the SNCP switching. Perform the test again. It is found that the switching is successful.
When a fiber cut occurs for the second time, check the SNCP switching status. The SNCP switching of NE1 is in the normal state, which is the reason why the SNCP switching does not occur. On this VC-4 link, NE1 reports the HP_UNEQ alarm to the XCL board and the T2000. Analyze the cross-connections of NE4. It is found that no cross-connection is created. ON NE5, the MSP protection paths are pass-through. Hence, it is determined that no cross-connection is created on NE4 and thus the problem occurs. All the VC-4 signals are unavailable and then the downstream NEs report the HP_UNEQ alarm.