thank you, I think you are right, so we did, map all switch and setup root switch connected to LAN firewall, now I know why I have this issue. this ware two separate network and was never connected to each other. but our new requirement so test team to have access development side.so we need to connect it.  By following cables I found that the switch we ware trying to connect to Development network was 4 hop down  from root switch. so as usual, bdpu will elect close path for root. I have move that cable close to root switch and issue got resolved. this was a lesson for me to understand Spanning-tree bpdu. process. that I will never forget. thanks for all your help.
I have strange issue. our all switches are PC62XX with firmware 3.3.11.2. we have existing network switches and working fine now we added another switches for our new development environment. but issue is when I connect out LAN switch to Development switch we loose our network and all network goes down. I don't have any loop on both switches. cause I am using only one cable to connect to each other switch. 
avoid switch become as root