iRule to disable a node when health check fail

Okay, completely untested in the real world, but perhaps something like this:

1. Create two identical pools (same members) and apply your monitor(s) to only one of them - the "status" pool.

2. Apply a generic cookie profile to your VIP and this iRule:

   when RULE_INIT {
        user-defined: debug enable/disable
       set static::lb_debug 1
   
        user-defined: application pool name
       set static::app_pool "lb_test_pool"
   
        user-defined: status pool name
       set static::status_pool "lb_test_status_pool"
   }
   when LB_SELECTED {
       if { $static::lb_debug } { log local0. "Selected: [LB::server]" }
       if { [LB::status pool $static::status_pool member [LB::server addr] [LB::server port]] eq "down" } {
           if { $static::lb_debug } { log local0. "Status pool node down - reselecting" }
           LB::reselect pool $static::status_pool
       }
   }
   when HTTP_RESPONSE {
       if { [HTTP::cookie exists "BIGipServer$static::status_pool"] } {
           if { $static::lb_debug } { log local0. "Rewriting persistence cookie" }
           set persistval [HTTP::cookie value "BIGipServer$static::status_pool"]
           HTTP::cookie remove "BIGipServer$static::status_pool"
           HTTP::cookie insert name "BIGipServer$static::app_pool" value $persistval
       }
   }
   

Change the name of the static::app_pool and static::status_pool variables to the names of your application and status pools, respectively.

So you have two pools: one applied to the VIP and another exactly identical pool with your health monitor(s) applied. On LB_SELECTED, if a) there's no existing persistence and b) the chosen member from the primary pool is marked "down" in the status pool, reselect a new (known good) node from the status pool. LTM will send a persistence cookie for the status pool and node, which you'll rewrite to re-attach persistence to the app pool.

Also note that this iRule does not account for complete mid-session node failures, but it may roll over gracefully.