vCenter High Availability (vCenter HA) protects vCenter Server against host and hardware failures. The active-passive architecture of the solution can also help you reduce downtime significantly when you patch the vCenter Server. You create a three-node cluster that contains Active, Passive and Witness nodes. 

vCenter Architecture Overview

A vCenter HA cluster consists of three vCenter Server instances. The first instance, initially used as the Active node, is cloned twice to a Passive node and to a Witness node. Together, the three nodes provide an active-passive failover solution. Deploying each of the nodes on a different ESXi instance protects against hardware failure. Adding the three ESXi hosts to a DRS cluster can further protect your environment.

When vCenter HA configuration is complete, only the Active node has an active management interface (public IP). The three nodes communicate over a private network called the vCenter HA network that is set up as part of the configuration. The Active node is continuously replicating data to the Passive node.

Active 1. Runs the active vCenter Server instance
2. Uses a public IP address for the management interface
3. Uses the vCenter HA network for replication of data to the Passive node.
4. Uses the vCenter HA network to communicate with the Witness node.
Passive 1. Is initially a clone of the Active node
2. Constantly receives updates from and synchronizes state with the Active node over the vCenter HA network
3. Automatically takes over the role of the Active node if a failure occurs
Witness 1. Is a lightweight clone of the Active node
2. Provides a quorum to protect against split-brain situations

vCenter HA Hardware and Software Requirements

ESXi1. ESXi 6.0 or later is required.
2. A minimum of three ESXi hosts is strongly recommended.
3. Each vCenter HA node can then run on a different host for better protection.
Management vCenter Server Your environment can include a management vCenter Server system, or you can set up your vCenter Server to manage the ESXi host on which it runs (self-managed vCenter Server)
vCenter Server 6.0 or later is required.
vCenter Server1. vCenter Server 6.5 or later is required.
2. Deployment size Small (4 CPU and 16GB RAM) or bigger is required to meet the RTO. Do not use Tiny in production environments.
3. vCenter HA is supported and tested with VMFS, NFS, and vSAN datastores.
4. Ensure you have enough disk space to collect and store support bundles for all three nodes on the Active node. 
Network connectivity1. vCenter HA network latency between Active, Passive, and Witness nodes must be less than 10 ms.
2. The vCenter HA network must be on a different subnet than the management network.
Licensing required for vCenter HA1. vCenter HA requires a single vCenter Server license.
2. vCenter HA requires a Standard license.

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