timer trigger event

You could certainly do something that's a variation on that. There's no reason that I can see, given your current requirements, to wait for the LB_FAILED event. If we're going to manually check the active_members count of the pool we know is configured for the VIP anyway, we can do that in the HTTP_REQUEST.

 
 when CLIENT_ACCEPTED { 
   set loop 0 
 } 
  
 when HTTP_REQUEST { 
   if {[active_members primarypool] == 0} { 
     if { $loop == 0} { 
       set first [clock seconds] 
       set loop 1 
     } elseif { [expr [clock seconds] - $first] >= 600} { 
       pool backuppool 
     } 
   } 
 } 
 

The problem with something like this is...what are you going to do with the requests coming in for that 10 minutes where all of the active members in your primary pool are down, but you don't want to send them to your backup pool yet? Are they just going to be rejected? Sent to the down servers? Discarded? Sent back a message from the LTM? That's the piece of this iRule that's still missing.

As it sits right now they're going to be sent to whatever the VIP's default pool is (likely primarypool in this case) and if all of those members are down, they're going to use the fallback host, or just get a timeout if none is configured. That's probably not ideal.

Let me know what it is you want to do with them and I'll help you get this updated to include that as well.

Colin