maintenance splash screen

Please try this irule.

      when LB_FAILED {
        if { [active_members [LB::server pool]] == 0 } {
             Log and direct the client to Maintenance Page
            HTTP::respond 200 content {
           
              
        Apology Page
         
            
         Sorry! This site is down for maintenance. 
     
  
             }
        }
    }

You can also upload a custom page via iFile and reference the page in an iRule as follows.

when LB_FAILED {
    HTTP::respond 200 content [ifile get maintenance.html]
    event disable all
    return
   }