Nimbostratus
NimbostratusYou can use the 'priority group' feature to accomplish having a "primary" and "secondary" group of servers. Simply have your "primary" servers in priority group 100 and secondary servers in priority group 50, for example. (Note that the priority groups are used in order of HIGHEST priority.) 'Min active members' determines when the next priority group of servers comes online to accept connections. Once the available members in priority group 100 falls below 'min active members', BigIP brings the next group online for connections. In your example, the servers in priority group 2 will be active first, then priority group 1 active should the 'min active members' in group 2 fall below the level you set. Some additional info is available at:
The 'ratio' setting is related to the load balancing method, and does not determine when priority groups are activated. It relates to the ratio of new connections each server in a pool will receive. A common example used for 'ratio' LB methods is servers having differing performance characteristics. Ratio can weight connections more heavily to some servers over others to allow more 'powerful' servers to get more connections. You have not indicated such a case in your description so I am not sure that a 'ratio' LB method is right for your environment. The round-robin LB method will work fine with priority groups if your servers have similar characteristics.
Hope this helps...?