I wonder if LB_FAILED is triggered when a new TCP connection is established from a client to the VIP without persistence and all nodes in the pool are disabled. This might be the easiest option (check in LB_FAILED if [active_members [LB::server pool]]==0).
If that didn't work, you could insert a session cookie or use the cookie insert persistence profile cookie to see if the client had already established a session. The downside to using a session cookie is that a new client could get sent to the pool at any time--hours or days after the cookie was set assuming the client doesn't close their browser.
Aaron
Nimbostratus
