HTTP::respond

I expect you're seeing this error as well in the ltm log file:

May 10 11:13:25 tmm tmm[19697]: 011f0007:3: http_process_state_header_xfer - Invalid action EV_SINK_HEADER during ST_HTTP_XFER_HEADERS

Per bl0ndie's post here, this is a bug noted in versions prior to 9.4:

(Click here)

It is fixed in 9.4. In the meantime, you can send a redirect using 'HTTP::fallback http://mysorrypage.example.com'.

Also, you could retry the pool if there are nodes up when LB_FAILED is triggered. I think something like this would work on 9.4 (or substitute HTTP::respond with HTTP::fallback for < 9.4):

when HTTP_REQUEST {
   set retries 0
}
when LB_FAILED {
   if {[active_members [LB::server pool]] > 0 && ($retries < 3)}{
      incr retries
      log local0. "Reselecting with active members: [active_members [LB::server pool]] and retries: $retries"
      LB::reselect
   } else {
       no nodes available in pool, so respond back to client with BIG-IP generated content
      log local0. "Client: [IP::client_addr] lbfailed"
      HTTP::respond 200 content "blah"
   }
}

Aaron