There are four sites A, B, C and D comprising STM-16 MSP Ring. Some low order services are configured between A and C via B. However, different VC-4 were used in A-to-B section and B-to-C section. Fiber cable cut between A & B, protection switching occured and services were normal. After a while, fiber between B & C also cut, site B was isolated, but services between A and C were also interruped!
R_LOS in the sectino where fiber was broken
No AIS alarm on the affected services
No AIS alarm appeared because of misconnection. Problem was solved after using same VC-4 in different sections.
If the squelching function is enabled, AIS alarms will appear, however, this function cannot restore the traffic by making new cross connects automatically.
Without squelching, AIS alarms may not appear due to misconnection.
Only OSN products and 10G (Metro 5000) support this function.
This function is for MSP ring and all the nodes in MSP should support it.
It is enabled in the most of the versions by default, (except OSN V1R2).
Configuration through T2000 and command line is given in the attached guide
VC-4 # 1 was being used between A and B for low order services, whereas, VC-4 # 2 was being used between B and C. Multipoint failure or single node failure resulted in misconnection. Using same VC-4 rectified the problem.
Misconnection due to node failure and multipoint fiber cut is not Huawei product defect but this is MSP inherent problem. (Also found in other vendors equipment)
According to ITU-T standards, Squelching function is developed to avoid misconnection.