What is Global Server Load Balancing (GSLB)?

Global Server Load Balancing (GSLB) is a technology which directs network traffic to a group of data centers in various geographical locations. Each data center provides similar application services, and client traffic is directed to the optimal site with the best performance for each client.  GSLB monitors the health and responsiveness of each site, and like Server Load Balancing, directs traffic to the site with the best response times.

Load Balancing is a general term which provides:

Server Load Balancing provides Load Balancing services to an array of servers, generally, located at a single site or data center.

GSLB provides Load Balancing to multiple sites or data centers including the following services:

How Global Server Load Balancing Works

Client systems route to network application servers based on the data center servers IP address(s). Clients get this IP address from the local ISP DNS servers.

GSLB acts as Internet DNS services and provides the IP address to local clients accessing data center services.  GSLB systems intelligently calculate the most appropriate site to direct client traffic. Client traffic is directed, or load balanced, to sites depending on policies configured in the GSLB system.

GSLB include the following services:

How Standard DNS Works

Since GSLB technology directs client traffic using the DNS protocol, we will step through the steps a network client uses to retrieve the IP address of a web service.  GSLB will perform the same function as a DNS server with additional intelligence.

When a web browser connects to a web server over the Internet, the following steps occur:

  1. A client web browser attempts to connect to the web site with the URL such as https://somewebserver.com
  2. The client queries the DNS servers provided by the local ISP
  3. The ISP DNS servers query the Internet Root DNS system for the DNS servers which are the Authoritative DNS Servers for SomeWebServer.com
  4. The Internet Root DNS servers return the IP address for the Authoritative DNS servers which is managed by the SomeWebServer.com operations
  5. The local ISP DNS servers then query the Authoritative DNS servers for SomeWebServer.com and receives the IP address
    • Making a query from a local DNS server to an Authoritative DNS server is called a recursive DNS query
  6. The web browser now has the IP address and can communicate with the web server

How GSLB Provides DNS

Since client traffic is routed to servers based on the IP address retrieved from DNS, DNS services can be extended to intelligently direct clients to any number of data center sites hosting services. GSLB systems act as Authoritative DNS servers, and as such can direct client traffic to any data center site hosting the application service.

When clients query for a DNS address, the GSLB system can intelligently calculate the most appropriate site to provide services. Each client DNS query can be responded to by the GSLB system and responses are provided on a client by client basis. GSLB systems can therefor provide the functionality of a Server Load Balancer over geographically distributed data center locations, anywhere in the world.

The following are the steps for a client web browser to connect to a multi-site web service with GSLB technology.  The client system is not aware of GSLB.

  1. A GSLB system is acting as the organization’s Authoritative DNS servers. It provides the responses to requests for the IP address of SomeWebSite.com
  2. The client web browser queries the local ISP DNS servers for the IP address
  3. The ISP DNS servers query the GSLB systems.  The GSLB calculates the most optimum site to direct the client system and replies with that sites server IP address.
    1. The GSLB system records the IP address of the remote ISP DNS servers. The ISP DNS servers provided to their customers are typically located geographically to the client systems.
    2. From the local DNS server address, GSLB can deduce the geographical location of the client
  4. Using Load Balancing algorithms, traffic control policies and the geographical location of the client, GSLB can intelligently direct the client to the most optimal site.
GSLB DNS Architecture
GSLB DNS Architecture

GSLB provides intelligent DNS Authoritative DNS services, interacting with local ISP DNS servers.  Multiple data center sites each have systems providing redundant DNS and GSLB services.

GSLB as a DNS Proxy

GSLB are typically implemented as a DNS proxy server. GSLB systems are generally not appropriate as core DNS servers. GSLB systems do not provide a complete implementation of DNS such as provided by Microsoft Active Directory and others.

GSLB use as a DNS Proxy

How DNS Proxy Works

Combined Server Load Balancers and GSLB

Application health and availability provided by Server Load Balancers (SLB) can be leveraged by GSLB systems.  SLB systems can send server and application performance and health data to GSLB systems.  This combination provides a power mechanism to provide application level disaster recovery.

Client requests can be redirected based on availability or health of individual application services.


Since the GSLB system records the client’s ISP DNS server address, using geo-location services, the GSLB system can:

Load Balancing

GSLB systems can monitor loading and site health when either paired with SLB systems or network monitoring and management systems.

GSLB systems record the number of client connections directed to each site and can calculated general loading for each site.  When GSLB systems are deployed to geographical regions, the network performances to local and remote data center sites and to individual clients can be monitored and used in GSLB load balancing calculations

Load Balancing can be based on multiple criteria:

Related Terms

How A10 Networks Can Help

A10 Networks Global Server Load Balancing features are incorporated into the Thunder platform which includes a broad set of security and networking technologies. The features included creates a robust GSLB solution. The Thunder platform includes a rich array of security features which protect the Internet facing GSLB systems.

The Thunder platform also includes the A10 Application Delivery Controller product providing advanced award-winning technologies.

A10’s GSLB functionality extends load balancing to a global geographic scale by offering a choice of DNS Proxy or DNS Server methods. A10’s GSLB adds another layer of availability and performance to applications with minimal impact to your existing DNS architecture.


Learn how a GSLB can optimize your datacenter. Read our solution brief “Get More from Your Enterprise Network”.



July 10, 2018

About Robert Keith

Robert has 30 years of experience in IT technology development and infrastructure management. He was the founder of several infrastructure ventures including Intellivence, MaxSP, Sentrik and most recently was the CTO of Iron Networks. As CTO of Iron Networks in San Jose, CA, he worked directly with many companies in the Silicon Valley to design and architect network, security, and cloud solutions. He worked directly with Microsoft engineering in the design of their cloud architectures including storage, Hyper-V, Systems Center and Virtual Networking. He also worked directly with Hortonworks to design a Hadoop deployment and management system using CentOS and many layered software packages. READ MORE