2ServerIron ADX Global Server Load Balancing Guide
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Global Server Load Balancing overview
1
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 algori thm to rotate the list o f
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 ServerIron 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
ServerIron ADX. After you enable the GSLB protocol, the GSLB ServerIron ADX finds the site
ServerIron ADXs using their IP management addresses, which you specify when you configure the
remote site information. The GSLB controller ServerIron ADX 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 ServerIron ADX switches. The GSLB controller communicates
with these ServerIron ADX switches designated as "site ServerIron ADX switches" 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 ServerIron ADX switches. After you enable the
GSLB protocol on site ServerIron ADX switches and configure the IP addresses of the site
ServerIron ADX switches on the GSLB ServerIron ADX, then the GSLB ServerIron ADX establishes
communication with the site ServerIron ADX switches.
The GSLB ServerIron ADX uses the GSLB protocol to learn the following information from the site
ServerIron ADXs:
•The VIPs configured on the site ServerIron ADXs and the health of the VIPs —The site
ServerIron ADXs report VIP additions and deletions asynchronously. Each time a VIP is added
to a site ServerIron ADX, the ServerIron ADX sends a message to the GSLB ServerIron ADX to
inform the GSLB ServerIron ADX of the change.