I don't have a VI 3.5 environment to verify this, but you could try looking at the advanced vpx setings for your vCenter, at least in vSphere there is a property called instance.id which seems to be unique even though I have separate independent vCenter ... though I can't really confirm if this can ever be true ... I'm wondering why instanceName/IP would not be unique? If it was not, you would have conflicts within your environment? I would hope that at a bare minimal those would be somehow unique and easily identifiable between two instances?
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William Lam
VMware vExpert 2009
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