iRule to parse status page and failover to secondary server/priority group

Under the members tab of a defined pool, click on the help section to get more detail. Essentially though, once you enable priority group activation from the drop down box, "type a number from 0 to 65535 that represents the minimum number of members that must be available in one priority group before the system directs traffic to members in a lower priority group". So if you want the lower priority members to be used only if all of the higher priority members are down, enter a 1 in the threshold field.