Doing IT Virtual

This post is content adapted from Chapter 11 of the Microsoft Virtual Server 2005 R2 Resource Kit.

Duplicate MAC Addresses

Of course, errors are generated when you configure and start two virtual machines with identical static media access control (MAC) addresses on the same Virtual Server host and virtual network.

If you use a single staging server to build a large number of virtual machines and change the MAC address configuration to static from dynamic for tracking purposes, the dynamic MAC address allocation system could generate a duplicate MAC address. If the Virtual Server host contains only a single physical network adapter, Virtual Server 2005 R2 assigns MAC addresses to virtual network adapters in the 00-03-FF-xx-xx-xx range, where the last two octets match the last two octets of the physical network adapter MAC address. For example, if the MAC address of the physical network card on your staging server is 00-16-31-53-32-68, Virtual Server will assign virtual network adapters a MAC address in the range 00-03-FF-xx-32-68.

If there are multiple network adapters in the physical server, the first 256 MAC addresses are allocated using the primary network adapter octet values, the next 256 MAC addresses are allocated from the second network adapter octet values, and this process continues until Virtual Server 2005 R2 has iterated through all the physical network adapters. If all network adapter octet values are exhausted, Virtual Server re-uses the first network adapter octet values.

Resolution

In general, there is no guarantee that dynamic MAC address allocation will be unique, even across multiple Virtual Server hosts. If you use a single staging server to generate virtual machines configured with static MAC address, you must institute a process to ensure that duplicate MAC addresses are identified and reconfigured prior to deployment. If you copy a virtual machine with a static MAC address to another computer that already has a virtual machine with an identical static MAC address, you must either manually or programmatically change the static MAC address of one of the virtual machines, or configure one or more of the virtual machines to use a dynamic MAC address. Virtual Server allocates a new dynamic MAC address in the following circumstances:

· A virtual machine is created.

· A virtual machine MAC address conflict is detected.

· A virtual machine is registered on a Virtual Server host.

Follow these steps to modify the virtual machine configuration to use a dynamic MAC address:

• Open the Virtual Server Administration Website.
• In Virtual Machines, click Configure and select the virtual machine from the list.
• In the virtual machine Configuration pane, click Network Adapters.
• In Ethernet (MAC) Address, select Dynamic.
• Click OK.

More Info: For additional information regarding the Virtual Server dynamic MAC address allocation algorithm, refer to http://support.microsoft.com/default.aspx/kb/888030.