Configure a backup load balancing virtual server

Last published : Sep 24, 2026
You can configure the NetScaler appliance to direct requests to a backup virtual server when the primary load balancing virtual server is DOWN or unavailable. The backup virtual server is a proxy and is transparent to the client. The appliance can also send a notification message to the client regarding the site outage.
The backup load balancing virtual server ensures minimal disruption when the primary method is unavailable, increasing the availability and reliability of the load balanced environment.
Note:
The backup virtual server continues to handle the existing connections, even after the primary virtual server is deleted or disabled.
You can configure a backup load balancing virtual server when you create it, or you can change the optional parameters of an existing virtual server. You can also configure a backup virtual server for an existing backup virtual server, thus creating cascading backup virtual servers. The maximum depth of cascading backup virtual servers is 10.
If you have multiple virtual servers that connect to two servers, you have a choice for what happens if the primary virtual server goes DOWN and then comes back up. The default behavior is for the primary virtual server to resume its role as primary. However, you can configure the backup virtual server to remain in control when it takes over. For example, you can sync the updates on the backup virtual server to the primary virtual server and then manually force the original primary server to resume its role. In this case, you can designate the backup virtual server to remain in control when the primary virtual server goes DOWN and then comes back up.
You can configure a redirect URL on the primary load balancing virtual server as a fallback when both the primary and the backup virtual servers are DOWN or have reached their threshold for handling requests. When services bound to virtual servers are OUT OF SERVICE, the appliance uses the redirect URL.
Backup LB Method is displayed if the following load balancing methods are selected:
  • Least connection
  • Least response time
  • Round robin
  • Least bandwidth
  • Least packets
  • Custom load
  • Least request
  • Static proximity
Note
If a load balancing virtual server is configured with both a backup virtual server and a redirect URL, then the backup virtual server takes precedence over the redirect URL. A redirect is used only when the primary and backup virtual servers are down.

To set a backup virtual server by using the CLI

At the command prompt, type:
set lb vserver <vServerName> -backupVserver <BackupVServerName> [-disablePrimaryOnDown <ENABLED|DISABLED>]
Example:
set lb vserver Vserver-LB-1 -backupVserver Vserver-LB-2 -disablePrimaryOnDown ENABLED

To set a backup virtual server by using the GUI

  1. Navigate to Traffic Management > Load Balancing > Virtual Servers, and open the virtual server.
  2. In Advanced Settings, click Protection, and select a backup virtual server.
  3. If you want the backup virtual server to remain in control until you manually enable the primary virtual server even if the primary virtual server comes back up, select Disable Primary When Down.
Note: Starting from NetScaler version 12.1 build 51.xx, the GUI displays the effective state of that server indicating if the backup is active or not.
The effective state of the current server can be one of the following:
  • UP – Indicates that the server is UP
  • DOWN – Indicates that the server is DOWN
  • UP (Backup Active) – Indicates that either the primary or the secondary virtual server is UP and traffic is directed to the backup virtual server.
  • DOWN (Backup Active) – Indicates that both the primary and the backup virtual servers are down and traffic is routed to the backup virtual server.
When the Disable Primary When Down option is enabled on the primary virtual server and the primary server goes DOWN and is back UP again, the traffic is still served by the backup virtual server until the primary virtual server is re-enabled explicitly. You can use the command enable lb vserver <vserver_name> command to re-enable the primary virtual server.