a vCenter "Datacenter" object is a boundary for both vMotion and Distributed vSwitches. So if you would like to migrate VMs between buildings / clusters those clusters will need to be part of the same Datacenter object. Also if you want to have similar networks on both sides and like to use Distributed vSwitched then you will need to have both clusters part of the same DC so that the VDS objects are pushed down.
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