Alteon Application Switch Operating System Application Guide
Global Server Load Balancing
710
Document
ID:
RDWR-ALOS-V2900_AG1302
Metric Preferences
Setting metric preferences enables the GSLB selection mechanism to use multiple metrics from a
metric preference list. GSLB selection starts with the first metric in the list. It then goes to the next
metric when no server is selected, or when more than the required servers is selected. The GSLB
selection stops when the metric results in at least one and no more than the required servers, or
after the last metric in the list is reached. For DNS direct-based GSLB, the DNS response can be
configured to return up to 10 required servers. For HTTP redirects based GSLB, the required server
is one.
The following metrics can be selected from the metric preference list:
•
Geographical preference
•
Network preference
•
Least connections
•
Response time
•
Round-robin
•
Random
•
Availability
•
Quality of service
•
Minmisses
•
Hashing
•
DNS local
•
DNS always
•
Remote
•
Persistence
Rules
A rule lets the GSLB selection mechanism use a different GSLB site selection metric preference list,
and rules can be set based for the time of day. Each domain has one or more rules. Each rule has a
metric preference list. The GSLB selection selects the first rule that matches the domain and starts
with the first metric in the metric preference list of the rule. For more information on rules, see
Configuring GSLB with Rules, page 730
GSLB Availability Persistence
The GSLB availability metric is used in GSLB rules to select a server exclusively when that server is
available. Should that server become unavailable, the next available server in a list is selected to
service requests. Availability is determined by a rank assigned to each server ranging from the
lowest score of 1 to the highest score of 48. Multiple servers can be scored the same.
Rules that use availability as the primary metric handle failures by selecting the server with the next
highest score compared to that of the server that failed, and begins forwarding requests to that
server. Should the server that failed become operational again, that server regains precedence and
requests are routed to it once more.
GSLB availability persistence allows the administrator to change the behavior of the availability
metric to reassign requests to a server that had previously failed because of its higher initial score.
With availability persistence enabled, a server that takes over after a failure is assigned the highest
possible availability value (48). This ensures that after the server that failed becomes operational
again, it cannot regain precedence from the recovery server. Should this new primary server fail, its
original availability value is restored and the next server in the list gains the high precedence.
