DNS Forward(cname)

DNS Cache is the fourth option in the expanded menu in your screenshot (right below 'Zones'). Have you created a dns cache ? If not, there won't be one by default, and the default DNS profile disables caching anyway.

The first thing to work out is what handled the query on the BigIP DNS (GTM). It could be GSLB, caching, zonerunner, dns express, or bind (named). Any of those could have returned a result.

If 100.66.7.253 is a DNS listener (DNS / Delivery / Listeners), then check the statistics for that listener, then under Profiles, select the DNS profile ('dns' by default), to see what process handed the request

If the box hasn't done anything else, then you'll see non-zero numbers against one of those processes. Update us when you find what it was

Given that you've configured a forwarder, you're probably expecting bind (named) to handle the request. Bind will remember the answer for as long as the TTL in the response says it can do so. You can clear this by restarting bind:

bigstart restart named