On December 8, 2015, VMware released VMware ESXi 5.5 patch ESXi550-201512001 (2135410). This patch is known as U3b and it contains general and security fixes, nothing special. Usually, you would install this update without notice. But this time, you should better take a look into the release notes of ESXi 5.5 U3b, before you install this update. This is taken from the release notes:
Note: In your vSphere environment, you need to update vCenter Server to vCenter Server 5.5 Update 3b before updating ESXi to ESXi 5.5 Update 3b. vCenter Server will not be able to manage ESXi 5.5 Update 3b, if you update ESXi before updating vCenter Server to version 5.5 Update 3b. For more information about the sequence in which vSphere environments need to be updated, refer, KB 2057795.
To avoid problems, you have to update your vCenter first, then update your hosts. This sequence is recommended by VMware and I recommend to take a look into KB 2057795 (Update sequence for VMware vSphere 5.5 and its compatible VMware products)
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