In customer's network we were required to do loop testing on 1xVC4-4c services. Service was configured on 6SL64:VC4#61-64. As services are concatenated i first tried to inloop on VC4#61 only and assumed to get loop on other end but it failed.
After that i In looped all the VC4s at the same time and problem and problem was resloved. In order to loop concatenated services you have to loop all the VC4s i-e VC4#61-64.
When we analysed the situation we found that C2 byte was only receiving on VC4#61 which is "HDLC/PPP Mapping" and other VC4s (VC4#62-64) were "Unequipped". Therefore we first tried to only In loop VC4#61. But in actual it does'nt works.
For concatenated services it is required to loop all vc4s at the same time inorder to get loop on far end.