There are two intersecting rings with MSP&MSP configuration. There were all OSN7500 nodes in the network. Refer to the attachment for clear picture. NED in the attached figure went power off due to some power issue. Suddenly there occured service interruption of many sites which were passing through NED.
1) R_LOS alarms observed on both ends of NED in DWDM network
2) Switching alarms occured on both ends of NED in OSN7500 ring
After further investigations and advance level help from HeadQuarter, it was found that services were down even they were configured as consistent VC4s because they were not configured as DNI (Dual Node Intersecting) services. Although HQ suggested that DNI is not a good idea to implement as it utilizes lot of resources and also its a complex technique to understand. In this case all passthrough and add/drop services will definitely get down. To protect them is to configure DNI.
Technically speaking there should be no service affecting in case of node failure when services are configured as consistent VC4s through out. It was checked that all those services which went down were also configured as consistent VC4s. If Add/drop services are down due to node failure, then its a normal routine. But pass-through services should not get down.
Consider a ring NE1--NE2--NE3--NE4--NE1
If there is a node failure of NE2, then there should be no service interruption if service pass through with same VC4 (like from VC4#m to VC4#m, not from VC4#m to VC4#n), it will be protected, otherwise it will down. And those services which are adding/dropping will ofcourse down.
In case of node failure of intersecting node in intersecting rings with MSP&MSP, all services which are passing-through and add/drop will get down if (no DNI is configured).