Which exact version of vCenter you're running? And this is a brand new vCenter or it is a old one? And the hosts and vCenter are on the same network? If not, check the following KB article: ESXi/ESX host disconnects from vCenter Server 60 seconds after connecting (1029919) | VMware KB
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Richardson Porto
Senior Infrastructure Specialist
LinkedIn: http://linkedin.com/in/richardsonporto
