212 ServerIron ADX Global Server Load Balancing Guide
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Advanced GSLB configuration for IPv6
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By default, the weighted site metric is disabled. When enabled, it is placed second in the GSLB
algorithm, after the server (host) health metric. You can change the metric order and enable or
disable other metrics, although we do not recommend this. For more information, refer to
“Changing the order of GSLB for IPv6 policy metrics” on page 206.
DNS response processing
When the weighted site metric is enabled, the GSLB ServerIron ADX selects an IP address
belonging to a partic ular site to be the best IP address in the DNS reply to a client. The client
subsequently makes an SLB request to that IP address.
Using the weighted site metric, the GSLB algorithm calculates a relative weight for each IP address
and selects the IP address with the least relative weight. The GSLB ServerIron ADX uses the
following criteria to calculate the relative weight of an IP address:
•The number of times the GSLB ServerIron ADX selected the IP address as the best IP address
to reply to a client
•The number of eligible IP addresses to be evaluated by the weighted site metric, and the
weights of sites to which they belong
•A calculated weight assigned to an IP address, based on the following criteria:
•If the IP address is a real server, then the calculated weight is zero
•If the IP address is a virtual IP (VIP), the weight is calculated based on the site the VIP
belongs to, the weight of the site, and the number of candidate VIPs belonging to the site
and being evaluated by the weighted site metric
If an IP ad dress h as a rel ative we ight of z ero, or if an IP a ddress belong s to a sit e that d oes not h ave
an assigned weight, the IP address is not selected as the best IP address for a client. Note that all
real servers have a relative weight of zero, as do VIPs that belong to sites with no assigned weights.
If two or more IP addresses have the same relative weight, or if all of the IP addresses have a
relative weight of zero, all of the IP addresses with the same relative weight are passed on to the
next step in the GSLB algorithm, where the process of selecting the best IP address continues.
Traffic distribution specifications
In general, DNS response selection counters are maintained per IP address, per domain name. For
example, suppose you configure three GSLB sites with assigned weights. All three sites host the
application www.gslb.com and sites New York and London also host ftp.gslb.com, as illustrated
below.
www.gslb.com

VIP 2001:DB8::1 belongs to San Jose with a weight of 50
VIP 2001:DB8::2 belongs to New York with a weight of 30
VIP 2001:DB8::3 belongs to London with a weight of 20
TABLE 20 Example weighted site metric configuration
IP address Configured weighted site metric Relative weighted site metric
San Jose 15 33% (15/45 * 100)
New York 20 44% (20/45 * 100)
London 10 22% (10/45 * 100)
Total 45 100%