Connection limit on virtual server and message back to client

try below iRule. Else you can also try by using FALLBACK HOST in http profile. You can enter a URL/VS-IP here so if the VS connection limits are reached it'll redirect clients to the URL/IP. And bind one iRULE which will just give response to clients e.g. - Service is unavailable

when RULE_INIT {

  set ::max_connections 500

  set ::html_content "Try After sometime"

}

when CLIENT_ACCEPTED {

  set over_limit 0

  if {$::active_connections >= $::max_connections } {

   set over_limit 1

  } 

}

when HTTP_REQUEST {

  if {$over_limit}{

   HTTP::respond 200 content $::html_content

  }

}

2. If you are using Fallback host option

when HTTP_REQUEST {

HTTP::respond 503 content "Service Temporarily Unavailable"

}