Exchange Server 2013 OAB Web Distribution Configuration in Exchange 2010 & 2013 Coexistence.

Hi,

We have OAB Issue for Exchange 2010 Users in our Organization.

We have Exchange 2010 and Exchange 2013 Coexistence.While Checking in Exchange Server 2010 Server OAB properties in Org Configuration--> Mailbox-->OAB Properties -->Distribution we are not seeing any CAS servers and Its Grayed Out.

Please let us know How to add Exchange 2010 CAS in Exchange 2013 Web distribution.