High Availability (HA) L4 Sync Options

This article relates to LoadMaster firmware version 7.2.52.

As of LoadMaster firmware version 7.2.52, if you select the Inter HA L4 TCP Connection Updates check box in the HA Parameters screen of the User Interface (UI), three new options appear:

  • L4 Sync Threshold: The minimum number of incoming packets that a connection must receive before the connection is synchronized. The range of the threshold is from 0 to (the Sync Period -1). The default value is 3.
  • L4 Sync Period: A connection is synchronized every time the number of its incoming packets modulus Sync Period equals the threshold. Valid values range from the Sync Threshold+1 to 255. The default value is 50.
  • L4 Sync Refresh Period: The difference (in seconds) in the reported connection timer that triggers a new sync message. Valid values range from 0-10. The default value is 0.

When the Sync Period and Sync Refresh Period are 0, syncs are only sent for state changes or only once when the packets match the Sync Threshold.

These settings might be useful in a scenario where a load balanced application is using an SSH session that requires a secure token. Adjusting these settings appropriately could prevent the SSH session from failing when a HA failover occurs.

There are some other settings that should also be configured:

  • The Switch to Preferred Server option should be set to No Preferred Server.
  • An SSH keep-alive should be configured either on the client side (for example, ServerAliveInterval) or server-side (for example, ClientAliveInterval).

Application Programming Interface (API) Details

You can run a get RESTful API command to retrieve or a set command to configure the following parameters:

  • L4SyncThreshold
  • L4SyncPeriod
  • L4SyncRefreshPeriod

For example, to retrieve a value:


To set a value:


For further details on the RESTful API in general, refer to the Long Term Support (LTS) RESTful API Interface Description document.

For PowerShell help, run the Get-Help command for the relevant commands.

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