Global Server Load Balancing (GSLB)
What this domain tests
GSLB tests your ability to direct DNS clients to the best data center, using ADC sites, GSLB vservers, services, and metric exchange. Expect questions on MEP, DNS delegation, and site failover.
Core rules
- A GSLB site represents a data center and has a site IP used for metric exchange protocol (MEP) communication between sites.
- A GSLB vserver is the DNS-facing object; GSLB services point to local or remote load-balancing vservers.
- MEP exchanges site health and load metrics over TCP port 3009 by default (and UDP 3009 for some traffic). Firewalls must allow it.
- Methods include round trip time (RTT), static proximity, geographic, and custom. RTT uses probes; static proximity uses configured location data.
- DNS delegation can be a CNAME from the public domain to the GSLB vserver, or NS delegation. The ADC can act as authoritative DNS or DNS proxy.
- Site persistence and backup GSLB vservers handle failure. A site is DOWN when its MEP heartbeats fail or the site is manually disabled.
Worked example
A company has GSLB sites in London and New York. The GSLB vserver uses RTT. A London client resolves the name and receives the London VIP. The London site's MEP heartbeat fails because port 3009 is blocked between sites. Reasoning: without MEP, each site cannot exchange metrics; the remote site may be marked DOWN and all DNS responses go to the surviving site. The fix is to open TCP/UDP 3009 between site IPs, not between NSIPs, and verify the site IP is reachable. After MEP recovers, RTT metrics repopulate and London clients can be sent locally again.
Traps that cost marks
GSLB does not load-balance packets; it returns DNS answers. The GSLB service port is often 80/443 but must match the target LB vserver. MEP uses site IPs, not NSIPs. A GSLB vserver can be UP while a site is DOWN. DNS TTLs control how long clients cache an answer, so failover is not instant. Do not confuse GSLB with global load balancing at layer 4.