An overview of how high availability works.
Summary
If you have two Application Switches, you can deploy them in a configuration where one Application Switch actively accepts connections and manages servers, while the second monitors the first. If the first Application Switch quits accepting connections for any reason, the second Application Switch takes over and begins actively accepting connections. This prevents downtime and ensures that the services provided by the Application Switch will remain available even if one Application Switch ceases to function.
The secondary Application Switch monitors the primary by sending periodic messages, or health checks, to the primary to determine whether it is accepting connections or not. If a health check fails, the secondary retries the connection for a specific time period until it determines that the primary Application Switch is not functioning normally. After making that determination, the secondary Application Switch takes over for the primary, a process called failover.
After a failover, all clients must reestablish their connections to the managed servers, but session persistence rules are maintained as they were before failover. If Web server logging is enabled after failover, it will remain enabled on the system that has taken over as primary. That is, no log data need be lost due to the failover. To enable logging persistence, the log server configuration must carry entries for both Application Switches in the log.conf file.
The following figure shows a network configuration with an HA pair. 
Application Switch in High Availability Mode
Note: Hubs can be used instead of switches. If hubs are used, check the interface and duplex settings on the Application Switch.
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