AU_AIS alarm was appearing on one of the STM which was configured on DWDM. Then DWDM network is connected with OSN3500. On OSN3500 SNCP was created in which working path was defined as DWDM and Protection path was defined as third party media. AU_AIS alarm was appearing on protection path which confirmed that DWDM media is clear and there may be issue with third party protection path.
SF_Condition on protection path of SNCP
After careful examine it was found that the cable between third party DDF and Optical/Electrical converter was damaged. After replacing it the problem was resolved and switching state of SNCP became "normal state".
1) Check service configuration on DWDM network and also on Metro NE OSN3500.
2) Check SNCP is configured correctly on OSN3500.
3) Check working and protection paths of SNCP created
Always check cables and ensure end-to-end connectivity in third party media is present.