Home Resource Center Glossary Global Server Load Balancing (GSLB)

Global Server Load Balancing (GSLB) is popular for its disaster recovery functionality as well as for more intelligent direction of traffic for optimal site selection.

A10's GSLB functionality is available in all the Thunder and AX Series Application Delivery Controller and Load Balancer products. Flexible options, seamless implementation and exceptional value complement the A10 GSLB benefits:

  • Provides data center failover and continuity
  • Optimizes multi-site deployments
  • Ensures user's Web experience is the fastest
  • Defines policy metrics with drag-and-drop GUI for ease of use and deployment
  • Includes geographic and network proximity policy metrics
  • Both DNS Proxy and DNS Server modes are supported
  • Run local traffic management and global traffic management on the same appliance

GSLB Deployment Modes

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 while allowing you to choose the method that best fits your environment:

DNS Proxy

  • Continues to use existing DNS infrastructure without changing the DNS server configuration.
  • Eliminates need to create or delegate sub-domains. Existing DNS maintains control.
  • DNS responses will be modified by the A10 ADC appliance as required by the configured policy. All other DNS responses will be passed through unchanged.
  • How it works: All DNS queries arriving at the DNS proxy IP address are forwarded to your existing DNS server.
    • The authoritative record for the zones on the root servers should point to the DNS proxy IP address on the A10 ADC.
    • No changes on the DNS server are required.

DNS Server (For A Domain)

  • In DNS Server mode, the A10 ADC appliance responds directly from its local GSLB DNS configuration for required DNS responses. The A10 ADC can be configured to respond as either authoritative or non-authoritative
  • How it works: The A10 ADC is set as the authoritative name server for an entire zone, for example "a10networks.com"
    • All requests will be processed and responded to by the A10 ADC, which directly responds to address queries for specific service IP addresses in the GSLB zone
    • The A10 ADC can still forward other types of queries to a DNS server
    • Support for multiple record types in addition to A and AAAA (IPv6) includes DNS MX, NS, PTR and SOA records
    • Note: A10 ADCs are not recommended as a full DNS server replacement

DNS Server (For A Sub-Domain)

  • In DNS delegation mode, the A10 ADC appliance responds directly for required DNS responses for a pre-defined delegated sub-domain
  • How it works: The A10 ADC appliance is set as the authoritative name server for a sub-domain. For example "gslb.a10networks.com"
    • All requests will be sent to the existing DNS authoritative infrastructure by the root servers, in this example "a10networks.com"
    • The existing DNS infrastructure will contain a subdomain for which the A10 ADC is authoritative. All requests for this sub-domain will be sent to the A10 ADC for resolution
    • All services that need GSLB will have a CNAME record in the primary domain replacing the existing A record, for example "www.a10networks.com IN A" becomes "www.a10networks.com IN CNAME www.gslb.a10networks.com"
    • Now user requests for www.a10networks.com will be resolved by the A10 ADC

Intelligent GSLB

GSLB functionality for applications can be leveraged through existing A10 ADCs deployments, combining local server load balancing and GSLB on a single box, or separately by running GSLB on its own dedicated A10 ADC platforms. All A10 ADCs participating in GSLB and local server load balancing communicate with one another and share the overall health information of the data center and applications.

By adding intelligence to DNS, application load can be based on many different factors, including:

Disaster Recovery

  • Provide an extra level of High Availability to important applications
  • Direct requests based on availability or health of the application

Active Round Delay Time

  • Send client connections to the fastest responding data center
  • Base response time on ping or DNS response

Single Sample Round Delay Time (Single Shot)

  • Take a single sample and use that sample indefinitely
  • Send a single DNS query to the GSLB local DNS


  • Send client connections to the "closest" physical data center
  • Integrate with geo-location services using any CSV format
  • Import third-party geo-location lists
  • Select to city level for additional granularity selection

Weighted Values

  • Send client connections to the data center that has the highest combined score
  • Send client connections to the data center with the most available active servers

Bandwidth Cost

  • Query the bandwidth utilization of each site
  • Select the site(s) whose bandwidth utilization has not exceeded a configured threshold during the most recent query interval