How to check what EVC mode you can use.

I started on this page-> http://www.vmware.com/resources/compatibility/search.php?scat=cpu&productid=68&deviceCategory=server&details=1&partner=41&releases=202&cpuSeries=68&page=1&display_interval=10&sortColumn=Partner&sortOrder=Asc

I had existing hosts with an older CPU Xeon X5660

http://www.vmware.com/resources/compatibility/detail.php?scat=cpu&productid=24&deviceCategory=server&details=1&partner=41&releases=202&cpuSeries=24&page=1&display_interval=10&sortColumn=Partner&sortOrder=Asc

They wanted to add hosts that had newer CPU Xeon E5-2670v2

http://www.vmware.com/resources/compatibility/detail.php?scat=cpu&productid=68&deviceCategory=server&details=1&partner=41&releases=202&cpuSeries=68&page=1&display_interval=10&sortColumn=Partner&sortOrder=Asc

So in this situation to be able to have hosts with both CPU types in the same cluster you would have to use an EVC mode of Westmere.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1003212 – This describes the EVC modes if you have someone who wants to know.