Brocade Virtual ADX Global Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 16)

6 Brocade Virtual ADX Global Server Load Balancing Guide
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Global Server Load Balancing overview
1
The client’s local DNS server might cache DNS replies from the authoritative server. Normally,
these cached responses would prevent the global SLB from taking place, since the local DNS
server would respond directly to the client without sending a recursive query to the authoritative
DNS server. However, the GSLB Brocade Virtual ADX, as a proxy for the authoritative DNS server,
automatically resets the Time-to-Live (TTL) parameter in each DNS record from the authoritative
server. By default, the GSLB Brocade Virtual ADX sets the TTL to 10 seconds. As a result, other DNS
servers that receive the records retain them in their databases for only 10 seconds. After the ten
seconds expire, subsequent requests from the client initiate another query to the authoritative DNS
server. As a result, the client always receives fresh information and the address of the site that is
truly the best site for the client.
NOTE
You also can change the TTL if needed. However, Brocade Communications Systems Inc.
recommends that you do not change the TTL to 0, because this can be interpreted as an error by
some older DNS servers.
You identify each ADX device by its management IP address, not by any VIPs configured on the ADX
device. Optionally, you also can specify a name for each ADX device at the site.
If a remote site is managed by one or more ADX devices, the GSLB Brocade Virtual ADX gathers
additional information from the site ADX devices using Brocade’s GSLB protocol with the remote
ADX devices. The protocol uses TCP port 182. To initiate the GSLB protocol between the GSLB
Brocade Virtual ADX and the ADX devices at the remote sites, you must first enable the GSLB
protocol on those remote ADX devices, then identify the sites and the ADX devices. In this example,
the GSLB Brocade Virtual ADX is configured with site information for Site 1 in Sunnyvale and Site 2
in Atlanta. Each site has servers containing the content for domain names within the domain
brocade.com. The servers are load balanced by the ADX devices.
GSLB policy
The Brocade Virtual ADX can use the following metrics to evaluate the server IP addresses in a DNS
reply:
• The server’s health
• The Weighted IP value assigned to an IP address
• The Weighted Site value assigned to a site
• The site ADX device’s remote SI session capacity threshold
• The IP address with the highest number of active bindings
• The geographic location of the server
• The connection load
• The site ADX device’s available session capacity
• The site ADX device’s FlashBack speed (how quickly the GSLB receives the health check
results)
• The site ADX device’s administrative preference (a numeric preference value you assign to
influence the GSLB policy if other policy metrics are equal)
• The Least Response selection (the site ADX device that has been selected less often than
others)
• Round robin selection (an alternative to the Least Response metric)