Consider the following scenario:
You deploy Microsoft Exchange Server 2013 and Exchange Server 2007 or Exchange Server 2010 in a coexistence environment.
Exchange 2013 HttpProxy tries to locate an earlier version client access server (CAS), such as an Exchange Server 2007 or Exchange Server 2010 CAS.
The load from Exchange 2013 HttpProxy to Exchange Server 2007 or Exchange Server 2010 CAS server isn't distributed equally. The pre-Exchange 2013 load is concentrated on a few servers, and other servers have almost no load.
In this scenario, HttpProxy routes to an earlier version client access server, and then that client access server is overloaded.
This issue occurs because hashing algorithm doesn't handle well for the Exchange server naming and Exchange server version.
To fix this issue, install Cumulative Update 12 or a later cumulative update for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.