VMware ESXi and vSphere Cluster Management

Enable vSphere Fault Tolerance for a Virtual Machine

Learn how to configure FT Logging on ESXi hosts, enable vSphere Fault Tolerance, and verify Primary and Secondary VM placement in vCenter Server.

What vSphere Fault Tolerance Does

vSphere Fault Tolerance (FT) protects a virtual machine from an ESXi host failure by maintaining a synchronized secondary execution instance on another compatible ESXi host. The active instance is the Primary VM; its synchronized counterpart is the Secondary VM.

FT uses vLockstep to keep the execution states of the Primary and Secondary aligned. If the host running the Primary fails, vSphere can continue the VM's execution on the Secondary host with minimal interruption, rather than waiting for a conventional restart.

FT is different from VMware High Availability (HA). HA normally restarts an affected VM on another host after a host failure. FT maintains a continuously synchronized Secondary VM so the protected workload can continue running through a protected host failure.

Plan the FT Configuration

Use vCenter Server to configure hosts and enable FT. The ESXi hosts must be managed by vCenter, and the administrator must have permissions to edit host networking and enable Fault Tolerance on the VM.

For the workflow in this lesson, power off the target VM before turning on FT. Confirm that the VM and its guest operating system are supported for the intended FT configuration. Also review the compatibility limitations for the VM's virtual hardware, devices, workload, and settings. A VM can be otherwise healthy and still be ineligible for FT because of an unsupported configuration.

Participating ESXi hosts must be compatible with one another and must have enough CPU, memory, datastore, and network capacity for both VM instances. The cluster must be able to place the Primary and Secondary on different hosts. Storage required by the VM must be shared or otherwise accessible to the eligible destination host according to the environment's storage design.

Before changing a protected VM's hardware, networking, storage, or protection settings, use supported vSphere procedures and check the resulting FT status afterward. Host maintenance and network changes can also affect synchronization.

For related preparation, review assigning permissions, vSphere alarms, and admission control.

Configure the Fault Tolerance Logging Network

Fault Tolerance Logging is an ESXi VMkernel network service used for synchronization traffic between the hosts running the Primary and Secondary. A VMkernel adapter is an ESXi host network interface used by host services such as management, vMotion, storage, and FT Logging.

FT Logging traffic needs reliable connectivity between participating hosts. Use an appropriately sized, low-latency network. Depending on the design and traffic requirements, separate FT Logging from unrelated traffic so other services do not compete with synchronization traffic.

Enable FT Logging on the first host

1. In the vSphere Client, select the first eligible ESXi host.
2. Open Manage, then select Networking.
3. Open VMkernel adapters.
4. Select the VMkernel adapter intended for FT traffic and choose Edit.
5. In the adapter's services, enable Fault Tolerance Logging.
6. Apply or save the change.

Repeat the configuration on every participating host

Apply the equivalent configuration on every ESXi host that may run the Primary or Secondary VM. In a two-host cluster, both hosts must have a VMkernel adapter with FT Logging enabled and a functioning path to the other host's FT Logging adapter.

• Confirm the intended VLAN is present end to end.
• Verify that each FT Logging adapter has the correct IP address and subnet configuration.
• Check the physical uplink, switch configuration, and any required routing.
• Verify reachability between the FT Logging interfaces on the participating hosts.
• Check bandwidth, latency, and packet loss before placing a critical VM under FT.

Enable Fault Tolerance on the Virtual Machine

1. Shut down the target virtual machine cleanly and confirm that it is powered off.
2. Locate the VM in the vCenter inventory.
3. Open the VM context menu or actions menu.
4. Choose All vCenter Actions > Fault Tolerance > Turn On Fault Tolerance.
5. Review the compatibility warning or confirmation prompt. Confirm the operation only after checking its implications and resource requirements.
6. Wait while vSphere selects an eligible host, creates the Secondary VM, and synchronizes it with the Primary VM.

During this operation, vSphere creates and places the Secondary VM on another eligible ESXi host. The operation can take time while storage is accessed, the secondary instance is established, and vLockstep synchronization reaches a healthy state. Do not treat the task as complete until the related task, FT status, and synchronization state indicate completion.

vSphere Client workflow:
Power off VM
  -> VM actions
  -> All vCenter Actions
  -> Fault Tolerance
  -> Turn On Fault Tolerance
  -> Confirm
  -> Wait for Secondary creation and synchronization

Verify Primary and Secondary Placement

After the enable operation completes, open the protected VM's Summary tab. Review the placement and status information.

Compare the Host and Secondary Location values. They must identify different ESXi hosts. Also check the FT status, synchronization state, recent tasks, and alarms associated with the VM. Confirm that the FT Logging adapters remain active and can communicate across the intended network.

Practical Example: Two-Host FT Cluster

Prepare the hosts

1. Select ESXi Host A in the vSphere Client.
2. Open Manage > Networking > VMkernel adapters.
3. Edit the selected VMkernel adapter and enable Fault Tolerance Logging.
4. Apply the equivalent FT Logging configuration to ESXi Host B.
5. Verify that both adapters communicate over the intended FT Logging network.

Expected result: Both hosts are prepared to exchange FT synchronization traffic and can host either the Primary or Secondary, subject to compatibility and capacity checks.

Protect the critical VM

1. Validate the VM's FT eligibility, storage access, host compatibility, and available capacity.
2. Power off the VM.
3. Open the VM actions menu and select All vCenter Actions > Fault Tolerance > Turn On Fault Tolerance.
4. Accept the confirmation prompt if it is presented.
5. Wait for vSphere to create and synchronize the Secondary VM.
6. Open the Summary tab and record the Host and Secondary Location values.

Expected result: FT is enabled, and the Primary and Secondary instances are placed on different ESXi hosts.

Interpret the placement

1. Open the protected VM's Summary tab.
2. Read the Host value. This is the Primary VM's host.
3. Read the Secondary Location value. This is the Secondary VM's host.
4. Compare the host names and confirm they are different.

Operational Concepts and Lifecycle

The Primary VM performs the active workload. The Secondary VM receives the synchronized execution information needed to continue the workload if the Primary host fails. FT availability therefore depends on more than the FT setting itself: host placement, cluster capacity, storage access, and the FT Logging network all matter.

• Keep enough compatible capacity for the Secondary and for normal cluster operations.
• Monitor FT status after enabling protection, host maintenance, networking changes, storage changes, or VM changes.
• Confirm that the Primary and Secondary remain on separate hosts.
• Recheck FT Logging services after changing VMkernel networking, switches, VLANs, uplinks, or routing.
• Follow supported procedures before changing virtual hardware, devices, networking, storage, or protection settings on an FT VM.
• Review vCenter tasks and alarms when synchronization becomes degraded or unavailable.

FT is not a substitute for backups, application-level resilience, or a disaster-recovery design. It addresses the specific availability problem of a protected VM losing its host.

Troubleshoot Common FT Problems

The Turn On Fault Tolerance option is unavailable or validation fails

• Confirm that the VM is powered off for this workflow.
• Check the VM's FT compatibility and virtual hardware settings.
• Verify that the guest operating system, devices, workload, and configuration are supported for the intended FT setup.
• Confirm that the VM is managed by vCenter and is located on eligible hosts.
• Verify that the administrator has the required vCenter permissions.

vSphere cannot create or place the Secondary VM

• Confirm that another compatible ESXi host is available.
• Check CPU, memory, datastore, and network capacity on the eligible hosts.
• Verify that storage required by the VM is accessible to the destination host.
• Check host compatibility, maintenance state, reservations, rules, and other placement constraints.

FT Logging validation fails or the Secondary does not synchronize

• Verify that Fault Tolerance Logging is enabled on a VMkernel adapter on every participating host.
• Check FT Logging VLANs, IP addresses, subnet configuration, routing, and physical uplinks.
• Look for inconsistent network settings between the hosts.
• Investigate bandwidth, latency, and packet-loss conditions.
• Confirm that network changes have not disabled or disconnected the intended VMkernel adapters.

The Primary and Secondary appear to share a host, or Secondary Location is missing

• Review the VM Summary tab and recent vCenter tasks.
• Confirm that a separate eligible host exists and is not in maintenance or an unusable state.
• Check resource exhaustion and placement constraints.
• Review FT status and related vCenter alarms.

FT becomes unhealthy after an environment change

• Review recent VM hardware, network, storage, or configuration changes.
• Confirm that FT Logging remains enabled and reachable on all relevant hosts.
• Check host availability and cluster capacity.
• Use supported procedures before making additional changes to the protected VM.

Key Takeaways

• FT protects a VM with a synchronized Secondary VM on another ESXi host.
• vLockstep keeps Primary and Secondary execution states aligned.
• vCenter Server is required for the described management workflow.
• Enable Fault Tolerance Logging on a VMkernel adapter on every participating host.
• Use reliable, appropriately sized FT Logging connectivity and verify VLAN, IP, uplink, routing, and reachability.
• Power off the VM, choose All vCenter Actions > Fault Tolerance > Turn On Fault Tolerance, and wait for synchronization.
• Use the VM Summary tab to compare Host and Secondary Location; the values should identify separate ESXi hosts.

For broader study, see the VMware ESXi online course, advanced vSphere HA options, and vMotion networking.