"F5 DNS-EXPRESS"

 Here is one of the key features of F5-DNS Module which is called "DNS-EXPRESS", in a nutshell  

The below diagram shows you how the "DNS-EXPRESS" feature could be used on F5-DNS Module to act as a 'Secondary DNS Server' by focusing on improving your DNS experience from "Client-side" (Hit Ratio = 100%), and removing the Query Handling Load on "Server-side" (Just AXFR/IXFR messages).
Furthermore, it provides relaying of "Notify/Zone Transfer" messages between your "Primary-NS" and other "LDNS Server(s)".
It should also be noted that in F5-DNS Flowchart, the "DNS-EXPRESS" feature has higher priority rather than "DNS-CACHE" or "DNS Load-balancing" options.