Scenario: A Citrix Administrator executed the command below in an active-active, global server load balancing (GSLB) setup. set gslb parameter…

The set gslb parameter –ldnsprobeOrder command configures the sequence of probes used for the dynamic proximity Global Server Load Balancing (GSLB) method. The purpose of these probes (DNS, PING, TCP) is to actively measure the network latency between the Citrix ADC and the client's local DNS (LDNS) server. This measurement of latency is known as the Round-trip time (RTT). The GSLB decision-making process then uses the collected RTT values to direct the user to the GSLB site (data center) that has the lowest RTT, thereby providing the best performance.

A. Time to live (TTL) is a value in an IP packet or DNS record that limits its lifetime; it is not a measure of network latency for proximity decisions.

B. Empty Domain Service (EDS) is not a standard GSLB or networking term and is irrelevant to what the specified probes are designed to measure.

C. Multiple IP responses (MIR) is a DNS feature where a server returns multiple IP addresses for a query, which is unrelated to measuring network proximity.

1. Citrix ADC 13.1 Product Documentation: In the section on GSLB

the dynamic proximity method is detailed. It states

"In the dynamic proximity method

the appliance selects the service that is closest to the client. The proximity is determined by measuring the round-trip time (RTT) between the client’s local DNS server and the DNS server for each GSLB site."

Source: Citrix ADC 13.1 Documentation

"Proximity-based GSLB methods

" Section: "Dynamic proximity method."

2. Citrix ADC 13.1 Command Reference: The documentation for the set gslb parameter command explicitly defines the ldnsProbeOrder argument. It specifies the "Order of LDNS probes to be used by the appliance to determine the RTT between the client's LDNS and the GSLB sites."

Source: Citrix ADC 13.1 Command Reference

"set gslb parameter

" Argument: ldnsProbeOrder.

3. Citrix ADC 13.0 Product Documentation: This version also confirms the behavior. "The dynamic proximity load balancing method determines the proximity of the client’s local DNS to each GSLB site by measuring the round trip time (RTT) between them."

Source: Citrix ADC 13.0 Documentation

"Global Server Load Balancing

" Section: "Proximity Based GSLB Methods."