Brocade Virtual ADX Global Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 52)

42 Brocade Virtual ADX Global Server Load Balancing Guide
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Configuring GSLB protocol parameters
1
DNS response processing
When the Weighted Site metric is enabled, the GSLB Brocade Virtual ADX selects an IP address
belonging to a particular 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 Brocade Virtual ADX uses the
following criteria to calculate the relative weight of an IP address:
• The number of times the GSLB Brocade Virtual 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 address has a relative weight of zero, or if an IP address belongs to a site that does not have
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.example1.com and sites New York and London also host ftp.example1com, as
illustrated below.
www.example1.com
VIP 10.1.1.1 belongs to San Jose with a weight of 50
VIP 10.1.1.2 belongs to New York with a weight of 30
VIP 10.1.1.3 belongs to London with a weight of 20
ftp.example1.com
VIP 10.1.1.2 belongs to New York with a weight of 30
VIP 10.1.1.3 belongs to London with a weight of 20
Suppose that 10 DNS requests are made to www.example1.com. By viewing the selection counters
(using the show gslb dns zone command), you would see that San Jose is selected 5 times (50%),
New York is selected 3 times (30%), and London is selected 2 times (20%).
Now suppose that 5 DNS requests are made to ftp.example1.com. In this case, New York receives
3 requests (60%), and London receives 2 requests (40%). This is because counters are maintained
per IP address per domain name.
If you consider the total site traffic for both applications, the traffic distribution is as follows: San
Jose = 5 (33%); New York = 6 (40%); and London = 4 (26%). The GSLB Brocade Virtual ADX
evaluates the results of the weighted metrics with respect to a specific domain name, not an IP
address alone.