Open Stack

Hi All,
I'm a newbie of keystone, and I'm doing some research about it recently. I have a question about how to deploy it. The scenario is on below:
 
One comany has one headquarter dc and 5 sub dc locate in different cities. We want to deploy separate OpenStack with "sub" keystone at the sub dc, and want to deploy one "master" keystone at headquarter dc. We want to manage all users, roles and tenants etc on the "master" keystone, however we want the end-user can authenticate with the "sub" keystone where he or she is locate.
  
Is anyone understant this scenario? How to realize it without additionaly development?
  
Thanks in advance!
 
Sam Cheng
 		 	   		  
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