Change LB priority based on status

neeeewbie I don't believe this is possible with just the LTM. I believe what you would really want is a GTM to server 2 virtual servers, one with server A in the pool and one with server B in the pool. Once you have that configuration in place you would set the GTM to balance DNS queries between the two sites and only give one site or the other if the opposing site virtual server failed. Maybe it's possible with TCL in an iRule but I don't believe anything exists that would have a running tally like you would need in order to flip back and forth between pool members.