Global Server Load Balancing (GSLB) and DNS
GSLB building blocks
GSLB answers DNS requests with a site-specific address. A GSLB site contains local GSLB services and communicates with remote sites through the metric exchange protocol (MEP). A GSLB vserver is the DNS-facing object that binds GSLB services and applies the GSLB load-balancing method. GSLB services can be local or remote; monitors bind to GSLB services. An ADNS service or DNS vserver receives the query, and delegation or a CNAME points the application name at the GSLB configuration. Site persistence can use source IP, cookie, or SSL session ID so a client remains at one site.
Methods and health
- Round Robin: rotates among eligible sites.
- Least Connection and Least Response Time: use site/service metrics.
- Static Proximity: uses a location database to choose the closest site by configured location.
- Dynamic Proximity: uses measured network proximity/RTT rather than a static database.
- Source IP Hash: deterministic client-to-site mapping.
Only healthy GSLB services should be selected. If a site's monitor fails, the method should choose another eligible site, and MEP should propagate site status. Backup IP and site persistence settings affect what happens when a site is unavailable.
Worked example
app.example.com is delegated to the ADC's DNS service. A GSLB vserver binds a local GSLB service in the EU site and a remote GSLB service in the US site. Static Proximity is configured with location data, so a client in Germany resolves to the EU address. The EU service monitor then fails. The GSLB vserver removes the EU service from selection and returns the US address; MEP carries the site status between peers. If the client had a site-persistence cookie for the EU site, the ADC must handle the down site rather than continue to return an unusable address. Without DNS delegation, clients never reach the GSLB vserver, no matter how correct the GSLB configuration is.
Traps
GSLB services are not ordinary LB services; monitors bind to GSLB services. Static Proximity requires location data; without it, the method cannot behave as intended. Source IP hash is not RTT-based. Blocked MEP traffic makes remote sites look unhealthy or stale. Forgetting DNS delegation is a complete failure even when GSLB objects are correct.