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Uniquely identifying VMs in vSphere Part 3: Enhanced Linked Mode & Cross VC-vMotion

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Back in 2012, I had published two articles which provides details and guidance on how to uniquely identify a Virtual Machine for both a vSphere and/or vCloud Director environment. The primary use case for this information was for customers or partners who have developed their own provisioning solution which requires them to track their VM assets throughout their lifecycle, usually in some sort of configuration management database (CMDB).

Although these articles are almost 5 years old, the content is still very relevant today and I still continue to reference them both with customers, partners and even some of our internal R&D folks. Most recently, I had a question about whether the guidance in these article were still applicable or whether they would be impacted by some of the new VMware technologies and capabilities that had been introduced since writing those articles such as Enhanced Linked Mode (ELM) and Cross vCenter vMotion (xVC-vMotion).

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Maximum number of vCenter Servers per Single Sign-On (SSO) Domain

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This particular question and its variations have been raised quite a bit lately by our field and customers. For me, this was an opportunity to see if we can provide some additional clarification and help explain some of the nuances that may have been causing some of the confusion around the supported maximums for both vCenter Server and the Platform Services Controller (PSC).

In the vSphere 6.5 Configuration Maximum, there are three specific maximums that helps us answer our question on the maximum number of vCenter Servers per vCenter Single Sign-On (SSO) Domain. I will go through each of the maximums and provide some additional context that will help us derive the answer to our question.

The first is the "Linked vCenter Servers" which defines the maximum number of vCenter Servers that can be supported in an Enhanced Linked Mode (ELM) configuration. What is interesting about this particular maximum is that it actually answers the majority of our question. By definition, an ELM consists of a single SSO Domain. This then means that you can only have a maximum of 10 vCenter Servers per SSO Domain.

vCenter Server Maximum

Configuration Maximum
Linked vCenter Servers (w/External PSC) 10
Linked vCenter Servers (w/Embedded PSC) 15

Note: As of vSphere 6.7, you can have up to 15 Embedded VCSA's within an ELM.

The second is the "Maximum PSCs per vSphere Domain" which defines the maximum number of PSC's that can be part of a single SSO Domain, pretty straight forward. The third is the "Maximum PSCs per site behind a load balancer" which just adds an additional constraint when using a load balancer with your PSCs.

Platform Services Controller Maximum

Configuration Maximum
Maximum PSCs per vSphere Domain 10
Maximum PSCs per site behind a load balancer 4

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How to split vCenter Servers configured in an Enhanced Linked Mode (ELM)?

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An interesting question that came up on the VMTN forum the other day (thanks to Andreas Peetz for sharing via Twitter) was how to split two vCenter Servers configured in an Enhanced Linked Mode (ELM)? Due to an organization changes in the customers environment, they needed to separate out their two vCenter Servers and run them independently of each other. Although this may sound like an rare event, I have actually seen this use case come up several times now which maybe from a business unit restructuring, spinning out or selling off company assets which then requires the customer to split their existing vCenter Servers that is configured with ELM.

Below is a diagram depicting an example where the original source environment (left) which is composed of two vCenter Servers and two external Platform Services Controller (PSC) configured in an ELM and the desired destination environment (right) which are two separate vCenter Server instances no longer configured in ELM.


The solution to this problem is actually pretty straight forward and leverages the existing vCenter Server and/or Platform Services Controller (PSC) "decommission" workflow. Rather than decommissioning the nodes, we are just simply keeping them around. Below are the instructions on how to achieve this outcome.

UPDATE (05/31/22) - I was recently made aware of the following VMware KB 2106736 article that provides official guidance for splitting/unregistering your vCenter Server from ELM. This should be followed as the officially supported method

UPDATE (01/28/19) - As of vSphere 6.7 Update 1, splitting an Enhanced Linked Mode (ELM) configuration is now supported by using the repointing workflow provided by the enhanced cmsso-util tool.

Disclaimer: Although this solution uses an existing supported workflow, this particular use case has not been tested by VMware. As such, this would not be officially supported by VMware until the appropriate testing has been done by our Engineering teams. One potential option in the short term if you are looking for support from VMware is to file an RPQ request through your VMware account team.

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