VMware is warning administrators to steer clear of an official update for Windows 7 and Server 2008 – after the patch was found to be incompatible with some virtual machines.

The virtualization house says that VMs using VMXNet3 virtual NICs are having networking problems after installing the Windows 7 rollup update.

VMs running Windows Server 2008 R2 SP1 are seeing similar problems, prompting VMware to recommend that anyone running either OS in their VMs on VMware vSphere should avoid installing the Microsoft rollup until a fix can be worked out.

"VMware is aware of this issue and we are actively investigating the root causes and possible fixes," the VMware blog says.

"While this effort progresses, VMware is advising customers to delay applying the Microsoft 'Convenience Update' to any virtual machine that uses the VMXNet3 vNIC type."

There is one saving grace in this mess; the rollup update is not being released through WSUS (Windows Server Update Services), and Microsoft says that the update is "optional," meaning admins need not worry about their VMs automatically downloading and installing a payload that will break their network connection.

Microsoft released the Windows 7 rollups last month in an attempt to simplify the process of getting updates for new installations. Designed to be used with either new PCs or install media, the rollup gives Windows 7 all previous updates rather than requiring the new machines to download and install hundreds of patches. ®