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.
The problem can be be avoided by two methods:
1.Always consider load balancing i.e. if there are 10 services from A to C, then five should be via B (A-B-D) and the rest five should be via D (A-D-C). In case of misconnection some services out of five a liable to be interrupted due to misconnection. So we can reduce the impact on services in case of failure of node B or D. Similar strategy can be adopted for other routes.
2.Use consistent VC-4 through out the path of a trail i.e. same VC-4 number should be used between A - B and B -C section. This helps to avoid misconnection which is inherent problem of MSP. There is a squelching function developed for OSN products but careful planning of services are very important.
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.
Service configuration planning is very important and above points should be kept in mind to avoid recurrence of these problem in future.