How to turn LTM 11.x licensed for DNS in an authoritative secondary DNS for a sub-domain?

I created a DNS Service Profile which I associated with my virtual server that is based on a sub-domain. Our organization DNS has been configured to send DNS requests towards our F5 box IP address. I associated an iRule to handle DNS requests and return the IP address of the node that is marked as being up:

 Send one or more IP addresses for a response to an A query

 Use on an LTM virtual server with a DNS profile enabled

when DNS_REQUEST {
     Log query details
    log local0. "[DNS::question name]: [DNS::question name],\
        [DNS::question class]: [DNS::question class], 
        [DNS::question type]: [DNS::question type]"
      if { [LB::server addr] equals "xxx.xxx.xxx.246" } {
     Generate an answer with two A records
    DNS::answer insert "[DNS::question name]. 111 [DNS::question class] [DNS::question type] xxx.xxx.xxx.246"
    DNS::return
      }
elseif { [LB::server addr] equals "xxx.xxx.xxx.247" }{
    DNS::answer insert "[DNS::question name]. 111 [DNS::question class] [DNS::question type] xxx.xxx.xxx.247"
    DNS::return
      }
     Stop further processing of the query after this iRule and send the answer to the client
}

It is not working. How can I troubleshoot this? Towards which IP address should I send secondary DNS requests from the primary DNS? The IP of the virtual server listener, or towards the GUI IP of the console, or else?