Forum Discussion
If I understand your requirement correctly, it sounds like priority group activation may solve your problem. For example, let's say you have a pool with four members (two pairs) assigned to a specific virtual server. Pair A would be the preferred priority group (with a priority of 5 and 4 assigned to the A members, respectively), and pair B would be the backup priority group (member priority of 3 and 2 respectively).
With this configuration, all requests would be directed to pair A primary member if it's available. If it becomes unavailable, then traffic would fail to pair A secondary member. Failure of both pair A members would mean traffic gets directed to pair B primary member.
http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-concepts-11-1-0/ltm_pools.html1216212