You are planning for a WANScaler implementation in your datacenter. For redundancy, you have multiple physical WAN Links and are planning to use the WANScalers in the simple “in-line” deployment in each one of the links.
While the WANScaler supports this configuration natively with the “group mode” feature set, network architects may wish to use an external link load balancing method instead. Depending on your network architecture, group mode can lead to additional traffic on the LAN side as network architects may not have the luxury of a separate network to handle the group mode related traffic .

This is where Citrix NetScaler can come to the rescue in a powerful way. NetScaler supports link load balancing capabilities that are well described in the product documentation. However, when designing for link load balancing with WANScaler in the picture, it is critical to ensure that the WANScaler appliances see all TCP segments associated with a connection in both directions. Therefore, special considerations need to be taken when designing link load balancing for WANScaler implementations:

(a) For connections initiated in the datacenter, it is critical that all TCP segments of the connections keep flowing over the same WAN link in both directions. This can be achieved by ensuring certain settings are applied (such as destination IP based persistency and the RNAT switch).

(b) For connections initiated from a branch office or a mobile user, the link load balancing decision must be made prior to the connection being actually established. This can be done by leveraging the DNS-based selection of NetScaler’s Global Server Load Balancing capability (although we’re not load balancing data centers in this example). Furthermore, once a selection is made by GSLB, the return packets must not be link load balanced, but must stick to the path selected in the GSLB step.

Sounds complicated? It’s not too bad and to make it easier for you, you can read all about it in the Consulting Solutions design considerations article published here.