Display a "We are overloaded page"

Hi.

Firstly you don't need your failover server, you can do that with a HTTP profile. There's "Fallback Host" in it, and when all pool members are unavailable it will send the request to this host (your failover server).

Regarding the difference between "all pool members unavailable" and "overloaded", LTM makes the difference.

To avoid that you have to use your own iRule to make a connection limit :

when RULE_INIT {
      set ::max_connections 100
      set static ::active_connections 0
}
when HTTP_REQUEST {
    if {$::active_connections > $::max_connections} then {
            HTTP::redirect "Your_newest_URL"
            TCP::close
            incr ::active_connections 1
    }
}
when CLIENT_CLOSED {
      incr ::active_connections -1 
}