Denis Fondras wrote:

...

What if primary.childzone.parentzone.example.com is the primary for parentzone.example.com, and childzone.parentzone.example.com?

In that specific case it looks like you are asking for trouble regardless of address family :)

What? It's a typical configuration with glues. For example, in my organization, ns1.noc.titech.ac.jp is the primary for noc.titech.ac.jp and titech.ac.jp. Masataka Ohta