Can I have 2 different ESXI Versions in the one Vcenter? MyVcenter is split up into several Clusters (for management and user separation/security reasons) . All host in a cluster are at the same patch level(EG 5.1 Update3). But they are not at the same level as the hosts in a different Cluster (EG 5.1 Update2 ). Is this OK or do I really need to update all the hosts?
You can have hosts with different ESXi versions managed by the same vCenter server. Up until last week, we had a couple ESXi 5.0.0 hosts managed by our 5.5.0u1 vCenter server, which also manages all our 5.5.0u1 hosts.
When I noticed the version difference, I upgraded the hosts ASAP, but we'd been in that state, with most of our hosts (and the vCenter server) on 5.5.0u1 and a couple hosts on 5.0.0, for at least several months.
In fact, having different ESX/ESXi versions in the same cluster is even supported - see this VMware blog post for details on that, including what not to do. It should serve as official confirmation that you can have different ESXi versions in the same vCenter, even if you're not planning on mixing ESXi versions in your cluster.