A vSphere upgrade will require careful assessment of the existing infrastructure. You will need to ensure that the server hardware is compatible with ESXi 6.5. If the existing infrastructure has vCenter components on Microsoft Windows, then you will need to verify whether the current Windows Servers versions are supported for the installation of vCenter 6.5 and its components. If the existing environment has other third layer components, such as the VMware NSX, vRealize Automation, vRealize Operations Manager, vCenter Site Recovery Manager, then it becomes essential to verify whether upgrading the core vSphere components will leave the third layer components unsupported/incompatible. In this section of the chapter, you will learn how to check the hardware compatibility of the existing server hardware, check Windows Server operating system compatibility with the vSphere 6.5 components, and verify third layer product interoperability with the core vSphere 6.5 management components. We will also review various upgrade paths available based on the existing vSphere environment's version and deployment models.

The following procedure walks you through the steps involved in planning a vSphere upgrade:

1. Hardware capacity and software requirements check: As with any new vSphere version, vSphere 6.5 does come with revised hardware capacity and software requirements. It is important to understand these requirements during the planning phase. VMware provides access to more than one form of reference material that would help you understand the hardware and software requirements to deploy a vSphere environment. One of the primary reference sources is the product documentation and in this case, the vSphere 6.5 Installation and Configuration guide (https://docs.vmware.com/en/VMware-vSphere/6.5/vsphere-esxi-vcenter-server-65-installation-setup-guide.pdf). Another source that is always kept up to date is VMware Knowledge Base (https://kb.vmware.com/s/).
2. Hardware compatibility check: The existing server hardware should be verified for its compatibility with ESXi 6.5. It is done by looking up the current server hardware's make and model in the VMware Compatibility Guide, which can be accessed at https://www.vmware.com/resources/compatibility.

3. vCenter component compatibility checks: The existing Microsoft Windows Servers hosting the vCenter components should be verified for its supportability with vCenter 6.5 and its components.
4. Product interoperability check: vCenter being the core management layer, there are other solutions that connect with vCenter through APIs to provide its services. Therefore, it becomes critical to verify whether the solution vendors, be it VMware or third party, has a vCenter 6.5 compatible solution yet. Also, consider upgrading the solutions and its plugins before you upgrade to vCenter 6.5. For instance, the first release of vSphere 6.5 did not add support for NSX. It was vSphere 6.5.0a that added support for NSX 6.3.
5. Upgrade paths: Depending on the current vSphere version and its deployment model, the process of upgrading to vSphere 6.5 could differ. Hence, it is important to understand the upgrade paths available. The oldest possible version that supports a direct upgrade is vSphere 5.5.
6. Download vSphere 6.5 components https://my.vmware.com/web/vmware/downloads: Download the vCenter Appliance or vCenter for Windows based on the platform decision that you have arrived at. Here is what you will need to download:
   • VMware vSphere Hypervisor (ESXi ISO) with VMware Tools
   • VMware vCenter and modules for Windows (ISO) or VMware vCenter Server Appliance (ISO)

The chances of successfully upgrading your vSphere environment without affecting the supportability and compatibility of your existing components will completely depend on how you plan and execute the upgrade. Once you have taken care of the hardware and software dependencies discussed in this section, you can safely execute the upgrade scripts to perform the upgrade. We will cover vSphere 5.5 to 6.5 and vSphere 6.0 to 6.5 upgrades in separate sections.