Let me explain how it works. Suppose we have two different clusters built with CPUs of different generations.
Cluster 1: G1, G2, G3 and G4.
Cluster 2: G3 and G4.
EVC level for Cluster 1 would be G1 - lowest level for oldest CPUs.
EVC level for Cluster 2 would be G3.
When you VMotion VM from Cluster 1 to host with G3 CPUs VM will continue to run fine because G3 CPU can all the stuff G1 can. The same thing if you VMotion VM to Cluster 2. But if you want VM to use all the new features that appeared after G1 you have to power VM off and start in G3 level cluster so new CPUID mask will be applied that would hide less features.
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MCSA, MCTS, VCP, VMware vExpert '2009
http://blog.vadmin.ru