Brocade Virtual ADX Global Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 12)

2 Brocade Virtual ADX Global Server Load Balancing Guide
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Global Server Load Balancing overview
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• Straight-forward configuration
• All IP protocols are supported
In standard DNS, when a client wants to connect to a host and has the host name but not the IP
address, the client can send a lookup request to its local DNS server. The DNS server checks its
local database and, if the database contains an Address record for the requested host name, the
DNS server sends the IP address for the host name back to the client. The client can then access
the host.
If the local DNS server does not have an address record for the requested server, the local DNS
server makes a recursive query. When a request reaches an authoritative DNS server, that DNS
server responds to this DNS query. The client’s local DNS server then sends the reply to the client.
The client now can access the requested host.
With the introduction of redundant servers, a domain name can reside at multiple sites, with
different IP addresses. When this is the case, the authoritative DNS server for the domain sends
multiple IP addresses in its replies to DNS queries. To provide rudimentary load sharing for the IP
addresses for domains, many DNS servers use a simple round-robin algorithm to rotate the list of
addresses in a given domain for each DNS query. Thus, the address that was first in the list in the
last reply sent by the DNS server is the last in the list in the next reply sent by the DNS server.
This mechanism can help ensure that a single site for the host does not receive all the requests for
the host. However, this mechanism does not provide the host address that is “best” for the client.
The best address for the client is the one that has the highest proximity to the client, in terms of
being the closest topologically, or responding the most quickly, and so on. Moreover, if a site is
down, the simple round-robin mechanism used by the DNS server cannot tell that the site is down
and still sends that site’s host address on the top of the list. Thus, the client receives an address
for a site that is not available and cannot access the requested host.
The Brocade Virtual ADX GSLB feature solves this problem by intelligently using health checks and
other methods to assess the availability and responsiveness of the host sites in the DNS reply, and
if necessary exchanging the address at the top of the list with another address selected from the
list. GSLB ensures that a client always receives a DNS reply for a host site that is available and is
the best choice among the available hosts.
Basic concepts
The GSLB protocol is disabled by default. You must enable the GSLB protocol on each site ADX
device. After you enable the GSLB protocol, the GSLB ADX device finds the site ADX devices using
their IP management addresses, which you specify when you configure the remote site information.
The GSLB controller front-ends the authoritative DNS server and provides the optimal IP address
for the querying clients. Some or all of the IP addresses in the DNS response reside on site ADX
devices. The GSLB controller communicates with these devices designated as "site ADX devices" in
order to exchange and obtain information needed to evaluate IP addresses contained in the DNS
responses.
The GSLB protocol is disabled by default on site ADX devices. After you enable the GSLB protocol on
site ADX devices and configure the IP addresses of the site ADX devices on the GSLB ADX device,
then the GSLB ADX device establishes communication with the site ADX devices.