Can happens when:
  • Each vCenter Server system has a vCenter Server instance ID. This ID is a number between 0 and 63 that is randomly generated at installation time, but can be reconfigured after installation.
  • vCenter Server uses the instance ID to generate MAC addresses and UUIDs for virtual machines. If two vCenter Server systems have the same vCenter instance ID, they might generate identical MAC addresses (duplicate MAC address) for virtual machines. This can cause conflicts if the virtual machines are on the same network, leading to packet loss and other problems.
  • When creating a new virtual machine, the MAC address is duplicated.
  • Virtual machines power on and function properly, but share a MAC address with another virtual machine.
  • This issue can occur if the amount of new virtual machines being deployed has exceeded the default pool of MAC addresses. This causes vCenter Server to start reusing already assigned MAC addresses.

The solution can be:

If you deploy virtual machines from multiple vCenter Server systems to the same network, you must ensure that these vCenter Server systems have unique instance IDs. When you change the vCenter Server Unique ID, the autogenerated MAC addresses will be different.
Note: A vCenter Server unique ID has 4 parts such as (This is not a MAC address):


The first three parts never change. When you change the vCenter Server unique ID with the steps in this article, only the fourth part changes.

To view or change the vCenter Server instance ID:

  1. Log in to vCenter Server using the vSphere Client, and select Administration > vCenter Server Settings.
  2. Select Runtime Settings.
    The vCenter Server Unique ID text box displays the current vCenter Server instance ID.
  3. If this ID is not unique, enter a new value between 1 and 63 in the vCenter Server Unique ID text box and click OK.
  4. If you changed the vCenter Server instance ID, you must restart vCenter Server for the change to take effect.

If you have existing virtual machines with conflicting MAC addresses, edit the MAC addresses to make them unique:

  1. Ensure that the virtual machine is powered off.
  2. In the vSphere Client inventory, right-click the virtual machine and choose Edit Settings.
  3. On the Hardware tab, select the virtual network adapter for the virtual machine.
  4. Under MAC Address, select Manual and enter a unique MAC address.
  5. Click OK.
  6. Restart the VMware VirtualCenter Server service. For more information, see Stopping, starting, or restarting vCenter services (1003895).
Alternatively, you can force vCenter Server to generate a new MAC address for the virtual network adapter by configuring the virtual network adapter to use a Manual MAC address, and then reconfiguring it to Automatic.
To verify if each host in the environment has a unique ID, run this command on each host:
esxcfg-info | grep -i “system uuid”