Brocade Virtual ADX Global Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 17)

Brocade Virtual ADX Global Server Load Balancing Guide 7
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Global Server Load Balancing overview
1
NOTE
The default order for the metrics is the order shown above.
The GSLB Brocade Virtual ADX evaluates each IP address in the DNS reply based on these metrics.
Based on the results, the GSLB Brocade Virtual ADX can reorder the list to place the IP address for
the “best” site on the top of the list.
If the GSLB policy rejects all of the sites, the GSLB Brocade Virtual ADX sends the DNS reply
unchanged to the client.
All of these metrics have default values but you can change the values if needed. In addition, you
can disable individual metrics or reorder them. Refer to
“Changing the GSLB policy metrics” on
page 33.
You also can configure the GSLB Brocade Virtual ADX to directly respond to DNS queries instead of
forwarding the queries to the authoritative DNS server and modifying the replies. Refer to
“DNS
cache proxy” on page 65.
The following sections describe each of these metrics in detail.
Server health
The GSLB Brocade Virtual ADX sends a Layer 3, Layer 4 TCP or UDP health check and Layer 7
application health check to the server to determine the health of the server and the host
application on the server. If the server fails either health check, the GSLB Brocade Virtual ADX
immediately disqualifies the server’s IP address from being the “best” site.
When you configure a Brocade Virtual ADX for GSLB, it learns a series of IP addresses from its
configured DNS real servers. Then it performs Layer 3, Layer 4, and if possible, Layer 7 health
checks against those IP addresses.
The GSLB Brocade Virtual ADX determines which health checks to use based on the host
applications you specify. For example, if a host name is associated with both HTTP and FTP
applications, the Brocade Virtual ADX sends the site Layer 4 TCP health checks (one for HTTP and
one for FTP) and also sends a separate Layer 7 HTTP health check and a separate Layer 7 FTP
health check. The site must pass all the health checks or it is disqualified from being the best site.
If a host application uses a port number that is not known to the Brocade Virtual ADX and
supported by GSLB, the Brocade Virtual ADX cannot perform a Layer 7 health check on the
application but still performs a Layer 4 TCP or UDP health check on the port. Health check
parameters such as retry interval, number of retries, and so on are global parameters.
NOTE
You can change the order in which the GSLB policy applies the metrics. However, Brocade
recommends that you always use the health check as the first metric. Otherwise, it is possible that
the GSLB policy will not select a “best” choice, and thus send the DNS reply unchanged. For
example, if the first metric is geographic location, and the DNS reply contains two sites, one in North
America and the other in South America, the GSLB policy favors the South American site after the
first comparison. However, if that site is down, the GSLB policy will find that none of the sites in the
reply is the “best” one, and thus send the reply unchanged.
NOTE
If all the sites fail their health checks, resulting in all the sites being rejected by the GSLB Brocade
Virtual ADX, the Brocade Virtual ADX sends the DNS reply unchanged to the client.