MAC duplication issues with captured VMs and WDS

I’ve previously reported problems with MAC duplication on Hyper-V host external network connections on Windows Server 2008 R2, which I’ve never fully resolved, although we have been successfully working around the issue as detailed in the first link above.

A couple of weeks ago I was working simultaneously on my Windows Server 2008 R2 laptop with Hyper-V (the same laptop build that’s been previously mentioned) and a Windows 7 x64 build that I was using for testing, when I noticed severe but intermittent network problems on both machines. After a fair amount of head scratching, I noticed that the two laptops had duplicated MAC addresses. Blatantly that shouldn’t happen, as the whole point of a MAC address is to provide uniqueness. The most perplexing issue was that the addresses conflicted across two different operating systems. However, it happened. Both wired adapters on the two machines had the MAC address 00-21-9B-DC-8E-0B. I uninstalled the wired adapter on the Windows 7 machine and scanned for new hardware. When the device reinstalled the problem went away.

Since then I’ve been poking around a bit and I’ve found this old TechNet article from December 2007 in which Wes Miller says:

In addition to changing the SID and the machine name, you also need to change certain values that may be specific to the virtual computing technology you’re using. In particular, you need to change the MAC address (the unique ID for networking devices). Plus, many virtual applications also have their own unique identifier. Most store these in their own machine configuration files, so you’ll want to know how to manipulate those entries (and maintain their validity). Note that many virtualization products that support Pre-Boot Execution Environment (PXE) key the SMBIOS UUID based on their own unique ID—emphasizing the need to change this (or let the virtualization software change it for you, if supported) if you’re joining it to a domain; otherwise, managing WDS or RIS-client systems can become impossible (if GUIDs conflict). Most of the virtualization solutions I’ve worked with can have severe networking problems in the case of duplicate MAC addresses; so if you are not just moving a virtual machine, it’s very important that you change the MAC address if the virtualization software does not do it for you.

That’s by no means conclusive, but I’m not turning up much else. So if you’re encountering network problems with WDS images that began their life as a virtual machine, you may want to consider automating the un/re-installation of the NICs post-deployment or addressing that with WAIK somehow. Otherwise just make sure to do it manually before giving the machine to the recipient, or people will wind up with a rather frustrating network issue.

If we get around to doing any more testing of this or gather more evidence from internal deployments, I’ll post the findings here. In the mean time I thought I’d update the ongoing saga here.

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