OWA: Integration of exchange 2013 cas into exchange 2010 environment

Hello together,

I hope someone of you already had this situation and is able to answer my questions.
Which steps are necessary to get a working OWA integration of ev in an environment with the coexistence of exchange 2010 (mbx) and 2013 (cas). 
Our customer plans to integrate the exchange 2013 cas servers first so that users are accessing their Exchange 2010 mailboxes over that. 

Tried to find a solution for this in my lab without success.
Are there any recommendations for this? 

 

Regards

Marc