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Citrix Global Server Load Balancing 101

Optimizing Load Balancing with Citrix's Global Server Load Balancing (GSLB)

Citrix is known for taking an existing technology and turning it into something better. XenApp has always been referred to as, “RDP on steroids” and with a click of a mouse, PVS allows dynamically deployable machines and servers.

Citrix’s Global Server Load Balancing (GSLB) is another technology following suit. As many of you know, DNS is an Internet service that translates domain names into IP addresses. Load balancers act as a reverse proxy and distribute network or application traffic across a number of servers to increase capacity and reliability of applications. For DNS, load balancing configures domains so client requests are distributed across a group of server machines. A domain can correspond to a website, a mail system, a print server, or another service that is made accessible via the Internet.

“DNS load balancing relies on the fact that most clients use the first IP address they receive for a domain. In most Linux distributions, DNS by default sends the list of IP addresses in a different order each time it responds to a new client, using the round-robin method. As a result, different clients direct their requests to different servers, effectively distributing the load across the server group.” –

Citrix developed a smarter service for load balancers using DNS. Just as DNS servers work, GSLB receives a DNS query and GSLB sends back an IP address. However, GSLB are able to do much more than traditional DNS servers:

  • Don’t provide an IP address unless it is UP (monitoring)
  • If the active IP address is down, it provides the passive IP address (active/passive)
  • Provides the IP address that is closest to the user (proximity load balancing)
  • Provides different IPs for internal and external (DNS View)

Use Cases for GSLB

Since we don’t want to remember IP addresses, all organizations have a need for DNS servers. Load balancers are also warranted because they are used to introduce a single name space with HA behind it.

Active and Passive IP Addresses

GSLB provides the best of both worlds. It really depends on the size of the enterprise and complexity of performance. Organizations may simply want to start with the Active/Passive function for automatic failover in a Disaster Recovery event. When an active IP address is down, GSLB will provide the passive IP address (active/passive). Many organizations will use this method so they don’t have to worry about DNS external or internal changes. To the user, everything stays the same and the “Passive” becomes the “Active.”

Proximity Load Balancing

For those in a global environment where bandwidth is a concern, check out proximity load balancing. Proximity load balancing is based on a user’s geographic location. When a user logs into the website a DNS query goes to the ADNS IP via their location (of which you’ll have to download onto your Netscalers). Traffic will be sent to the closest Netscaler to your users’ geographic location.

Internal and External IPs (DNS View)

The final GSLB feature I want to touch on is the internal or external IPs. The Netscaler decides which IP you go to based on source network. To the user, it’s a single DNS name but in the backend, a query is sent to the Netscaler and filtered through the Firewall or internal directory before requesting a server dynamically.

For more information on Citrix’s Global Server Load Balancing (GSLB), Netscaler, site redundancy and end user virtualization, contact one of our friendly IT consultants.

Written by: Syed Farid