A Load Balancer such as the Citrix NetScaler has monitors built in, to automatically redirect Client requests to the back-end servers that are up and running. But, what happens if the front-end is down? Load Balancing can fail when a Virtual Server or Vserver fails, or when the VIP is unable to handle excessive traffic. Protecting the LB setup against failure helps ensure the availablility of the websites and applications.

Diverting Excess Traffic to a Backup Load Balancing Virtual Server

The spillover option diverts new connections to a backup virtual server when the number of connections to the primary virtual server exceeds the configured threshold value. The threshold value is dynamically calculated, or you can set the value. The number of established connections (in case of TCP) on the virtual server is compared with the threshold value. When the number of connections reaches the threshold, new connections are diverted to the backup virtual server.

You can configure persistence with spillover. In this configuration, connections are diverted to the backup virtual server based on the persistence settings configured on the backup virtual server. These connections are not moved back to the primary virtual server after the number of connections drops below the threshold. Instead, the primary virtual server accepts new client connections.

If the backup virtual servers reach the configured threshold and are unable to take additional load, the primary virtual server diverts all requests to the redirect URL. If a redirect URL is not configured on the primary virtual server, subsequent requests are dropped.

With RTSP virtual servers, the NetScaler uses only data connections for spillover. If the backup RTSP virtual server is not available, the requests are redirected to an RTSP URL and an RTSP redirect message is sent to the client.

To set a primary virtual server to divert new connections to a backup virtual server by using the NetScaler command line

At the NetScaler command prompt, type:

  • set lb vserver <vServerName> -soMethod <spillOverType> -soThreshold <positiveInteger> -soPersistence ENABLED -soPersistenceTimeout <positiveInteger>


set lb vserver Vserver-LB-1 -soMethod Connection -soThreshold 1000 -soPersistence
enabled -soPersistenceTimeout 2

To set a primary virtual server to divert new connections to a backup virtual server by using the GUI configuration utility

  1. In the navigation pane, expand Load Balancing, and then click Virtual Servers.
  2. In the details pane, select the virtual server for which you want to configure the spillover (for example, Vserver-LB-1), and then click Open.
  3. On the Advanced tab, in the Method list, select the type of spillover, and in Threshold text box, type the threshold value (for example, Connection and 1000).
  4. Under Spillover, select the Persistence check box, and in Persistence Time-out (min) text box type the time-out (for example, 2).
  5. Click OK.

    Detailed instructions can be found in the NetScaler Traffic Management Guide

    NetScaler Documentation can be found here

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