Why is deploying four NetScaler appliances - two in the primary and two in the secondary data center with two NetScaler virtual servers for each appliance, each binded to a redirection policy, the bes

While the design aims for security and reliability, 'cost-effective' is not a primary or guaranteed benefit directly derived from this specific high-availability and traffic optimization architecture compared to other options.

Deploying NetScaler appliances in both primary and secondary data centers inherently offers redundancy and disaster recovery capabilities. The use of redirection policies, typically implemented via Global Server Load Balancing (GSLB), allows the system to direct users to the optimal or available data center, thereby optimizing external WAN traffic by reducing latency and improving resource utilization.

The design describes multi-datacenter deployment with virtual servers and redirection policies, which primarily focuses on redundancy and traffic steering, not the segmentation of Gateway and load-balancing features within or across appliances.

While NetScaler provides SSL VPN and the design might allow expansion, these are general capabilities of NetScaler and not the core technical reason this specific multi-datacenter and redirection policy design is considered best for redundancy and WAN optimization.