F5 DNS Forwarding

Hi Experts,

I have a domain, for example, testsite.com in zone.

Now, I would like to configure some specific domain to external dns server for lookup, and when client ask F5 to resolve, it will forward to external dns.

For example, when f5 receives request abc.testsite.com, and it will forward to external dns server.

I have configure zone list with FORWARD type and DNS cache, but seems not working, Any hints to configure it?