# Configure a backup unit or a connection synchronization for external high availability

You can now for an engine, configure a backup unit or configure a connection synchronization for external high availability.

##

Limitations:

- You cannot enable both the Backup Unit and the Connection Synchronization for External High Availability feature at the same time.
- This is only supported for SMC managed appliances.

The Backup Unit feature is a high availability functionality that is used to manage small remote sites where engine clustering is not possible or only a single IP address can be used. This functionality allows synchronization of the connection between two engine nodes and only one node is active at a time. Hence, if one of the engine nodes is unavailable the engine operations are not interrupted as the second engine node becomes active.

The Connection Synchronization for External High Availability feature is used to enable high availability in Public Clouds by allowing synchronization of connections between 2 to 16 single engine configurations.

## Steps

1. Select Engine.
2. Browse to Engine > Engines.
3. Right-click an engine, then select Edit <element type>.
4. In the navigation pane on the left, browse to Advanced Settings > High Availability.
5. Configure the settings.
6. Click Save.

## Engine Editor > Advanced Settings > High Availability

Use this branch to configure a backup unit or a connection synchronization for external high availability for an engine.

Option | Definition
--- | ---
Backup Unit section
Enable | Check or uncheck the Enable checkbox to enable or disable a backup unit for the engine. Note: When you disable the backup unit, factory reset of the engine that is used as a backup is automatically done and then it is powered off. Also, the license that was bound to the backup unit is now set to Unassigned status. You can choose to switch the engine active node. For more information, refer to the Switch Active Node topic. For details on license validity check and license status check of an engine, refer to the Check validity and status of licenses topic. For details on how to bind a license to a backup unit, refer to the Install licenses for unlicensed components topic.
Proof Of Serial for Pairing | Enter the Proof Of Serial (POS) of the appliance.
Connection Synchronization for External High Availability section
Enable | Check or uncheck the Enable checkbox to enable or disable the connection synchronization for high availability feature.
Interface for Unicast State Synchronization | Select an engine interface for the synchronization.
Synchronization Group | Select a synchronization group for the engine. Also, you can create a new synchronization group if needed. To create a new synchronization group: From the Synchronization Group drop-down list, select New. Enter a name for the group in the Name field. Optional. Enter a comment in the Comment field. Optional. Select the Select button to select a category. Click OK. Note: To add an engine to a synchronization group, you must navigate to the Engine Editor > Advanced Settings > High Availability section of the engine to enable the Connection Synchronization for External High Availability, and then select the group. This needs to be done for each engine individually.