vSphere 6.5 finally brought us vCenter High Availability which provides failover/RTO such that users can continue with their work in around 2 minutes through
API clients and in my observations around 5ish minutes through UI clients, awesome right!

vCenter HA is really simple to configure through the Wizard with a basic and advanced option, with the following requirements:

• vCenter Server 6.5 Appliance with internal or external PSC
• vCenter Server 6.5 Appliance with no snapshot and CD/DVD drive set to Client Device.
• Cluster with at least 3 ESXi hosts that this vCSA is both managing and running on will provide Basic installation option
• If your vCSA is managed by another vCenter Server that is not part of the same SSO domain, or running on a host that is does not manage, you will have to use the Advanced installation option.
• A port group on ESXi for the private HA network. Optionally, you can have a dedicated vSwitch if network isolation is a requirement.
  • The HA private network must reside on a different subnet than what is used for management.
• Network latency on the HA network must be less than 10ms.
• No gateway for the HA network must be specified when configuring the nodes manually during advanced installations.
• Storage:
  • Datastores for peer and witness cannot resides in a storage cluster.
  • Will get warning if you put nodes in same datastore

When vCenter HA is enabled, a three-node vCenter Server cluster (Active, Passive, and Witness nodes) is deployed, which consists of one IP address for management and three private IP addresses for the HA nodes.

1. Open vCenter Server 6.5 web client (HTML5 client does not yet support vCenter HA).
2. Select the vCenter server in Navigator
3. Select Configuration -> Settings -> vCenter HA
4. 
5. Click Configure

Basic install:

As mentioned in order to use the Basic installation your vCSA needs to run on a host that is managed by the current vCenter Server.

Prerequisites:

• Port group for vCenter Server HA network needs to be created.
• SSH must be enabled on the vCSA

1. 
   • Select Basic and Click next
2. 
   • Enter the HA IP address for the active node
     1. Remember this needs to be on a different subnet than your management IP address of the vCenter Server
   • Select the Standard or Distributed Port Group where the HA network resides
   • Click Next
3. 
   • Enter the HA IP address for the Passive and Witness node
   • If you need to override the failover management IP address for the passive node, you click on the Advanced button.
   • Click Next
4. 
   • In my case is received compatibility error due to my passive and witness nodes running on datastores that reside in a Storage cluster.
   • 
   • Click Edit
   • In the deployment configuration, select a datastore that does not reside in a cluster.
   • Click Next
5. 
   • Verify settings and click Finish
6. Your passive and witness nodes will be deployed automatically.
7. To verify that the vCenter HA cluster is active and functioning correctly
   • Select Configuration -> Monitor -> vCenter HA
   • 

Advanced install:

Prerequisites:

• Port group for vCenter Server HA network needs to be created.
• Manually add a second NIC to the vCenter Server Appliance
  • Attach it to the vCenter Server HA network port group that was create
  • Configure the vCenter Server HA IP address
• SSH must be enabled on the vCSA

1. 
   • Select Advanced
   • Click Next
2. 
   • Enter the HA IP address and Subnet Mask for the Passive and Witness node
   • If you need to override the failover management IP address for the passive node, you click on the Advanced button.
   • Click Next
3. 
   • Leave the wizard window open and manually perform the clone of the active vCSA
     1. Create a Passive node clone
        • The name of the VM in vCenter server =  “<vCSAname>-01”
        • For Clone options select
          • Create a new customization specification
          • Power on VM after creation
        • Computer name = Guest operating systems name is the same across all 3 nodes. Input same name of original vCSA VM
        • For NIC1 manually set the same management IP address as active vCSA
        • For NIC2 manually set the vCenter HA IP address
     2. Create a Witness node clone
        • The name of the VM in vCenter server =  “<vCSAname>-01-witness”
        • For Clone options select
          • Create a new customization specification
          • Customize this virtual machine’s hardware
            1. Set vCPU = 1
            2. Set Memory = 1GB
          • Power on VM after creation
        • Computer name = Guest operating systems name is the same across all 3 nodes. Input same name of original vCSA VM
        • Leave NIC1 as default (DHCP) since never used.
        • For NIC2 manually set the vCenter HA IP address
4. Click Finish
5. To verify that the vCenter HA cluster is active and functioning correctly
   • Select Configuration -> Monitor -> vCenter HA
   • 

Links:

The Basic walkthrough is available here.

The Advanced walkthrough is available here.

VMware docs available here.