Something that I really enjoy when I get a chance to, is to speak with our field folks and learn a bit more about our customer environments and some of the challenges they are facing. Last week I had quick call with one of our TAMs (Technical Account Managers) regarding the topic of Virtual Machine migration between vCenter Servers. The process of migrating Virtual Machines between two vCenter Servers is not particularly difficult, you simply disconnect the ESXi hosts from one vCenter Server and re-connect to the new vCenter Server. This is something I have performed on several occasions when I was a customer and with some planning it works effortlessly.

However, there are certain scenarios and configurations when migrating VMs between vCenter Servers that could potentially cause Virtual Machine MAC Address collisions. Before we jump into these scenarios, here is some background. By default, a Virtual Machine MAC Address is automatically generated by vCenter Server and the algorithm is based on vCenter Server's unique ID (0-63) among few other parameters which is documented here. If you have more than one vCenter Server, a best practice is to ensure that these VC IDs are different, especially if they are in the same broadcast domain.

As you can imagine, if you have two vCenter Servers that are configured with the same VC ID, there is a possibility that a duplicate MAC Address could be generated. You might think this is probably a rare event given the 65,000 possible MAC Address combinations. However, it actually happens more frequently than you think, especially in very large scale environments and/or Dev/Test for continuous build/integration environments which I have worked in the past and I have personally seen these issues before.

Going back to our vCenter Server migration topic, there are currently two main scenarios that I see occurring in customr environments and we can explore each in a bit more detail and their implications:

• Migrate ALL Virtual Machines from old vCenter Server to new vCenter Server
• Migrate portion of Virtual Machines from old vCenter Server to new vCenter Server

Migrate all Virtual Machines:

In the diagram above, we have vCenter Server 1 and vCenter Server 2 providing a before/after view. To make things easy, lets say they have VC ID 1 and 2. If we migrate ALL Virtual Machines across, we can see their original MAC Addresses will be preserved as we expect. For any new Virtual Machines being created, the 4th octet of the MAC Address will differ as expected and the vCenter Server will guarantee it is unique. If you want to ensure that new Virtual Machines keep a similar algorithm, you could change the vCenter Server ID to 1. No issues here and the migration is very straight forward.

Migrate A portion of Virtual Machines:

In the second diagram, we still have vCenter Server 1 and vCenter Server with unique VC IDs. However, in this scenario we are only migrating a portion of the Virtual Machines from vCenter Server 1 to vCenter Server 2. By migrating VM2 off of vCenter Server 1, the MAC Address of VM2 is no longer registered with that vCenter Server. What this means is that vCenter Server 1 can potentially re-use that MAC Address when it generates a new request. As you can see from the above diagram, this is a concern because VM2 is still using that MAC Address in vCenter Server 2, but vCenter Server 1 is no longer aware of its existence.

The scenario above is what the TAM was seeing at his customer's site and after understanding the challenge, there are a couple of potential solutions:

1. Range-Based MAC Address allocation - Allows you to specify a range of MAC Addresses to allocate from which may or may not helpful if the migrated MAC Addresses are truly random
2. Prefix-Based MAC Address allocation -  Allows you to modify the default VMware OUI (00:50:56) which would then ensure no conflicts would be created with previously assigned MAC Addresses. Though this could solve the problem, you potentially could run into collisions with other OUI's within your environment
3. Leave VMs in a disconnected state - This was actually a solution provided by another TAM on an internal thread which ended up working for his customer. The idea was that instead of disconnecting and removing the ESXi host when migrating a set of Virtual Machines, you would just leave it disconnected in vCenter Server 1. You would still be able to connect the ESXi host and Virtual Machines to vCenter Server 2 but from vCenter Server 1 point of view, the MAC Addresses for those Virtual Machines are seen as in use and it would not be reallocated.

I thought option #3 was a pretty interesting and out of the box solution that the customer came up with. The use case that caused them to see this problem in the first place is due to the way they provision remote environments. The customer has a centralized build environment in which everything is built and then shipped off to the remote sites which is a fairly common practice. Since the centralized vCenter Server is not moving, you can see how previously used MAC Addresses could be re-allocated.

Although option #3 would be the easiest to implement, I am not a fan of seeing so many disconnected systems from an operational perspective as it is hard to tell if there is an issue with the ESXi host and Virtual Machines or because it has been migrated. I guess one way to help with that is to create a Folder called "Migrated" and move all disconnected ESXi hosts into that folder which would help mask that away and disable any alarms for those hosts.

Some additional per-requisite checks that you can perform prior to the partial Virtual Machine migration:

Ensure that the destination vCenter Server is not configured with the same VC ID else you can potentially run into duplicate MAC Address conflicts. You can do this either manually through the vSphere Web/C# Client or leveraging our CLI/API to do so.

Here is an example using PowerCLI to retrieve the vCenter Server ID:

Get-AdvancedSetting -Entity $server -Name instance.id

Ensure no duplicate MAC Addresses exists by comparing the MAC Addresses of the Virtual Machines to be migrated to the Virtual Machines in the new environment. Again, you can either do this by hand (which I would not recommend) or leveraging our CLI/API to extract this information.

Here is an example using PowerCLI to retrieve the MAC Addresses for a Virtual Machine:

Get-VM |  Select-Object -Property Name, PowerState, @{"Name"="MAC";"Expression"={($_ | Get-NetworkAdapter).MacAddress}}

If there are other scenarios or solutions that you have seen with Virtual Machine migrations between vCenter Serves, feel free to leave a comment. I am sure others can benefit from past experiences or any other lesson learns.