How common is lack of DNS server diversity?

> > More interestingly, how many root servers allow recursive lookup?
> a quick looping probe shows that none of them do, nor the gTLD servers
> (phew!) although L.ROOT-SERVERS.NET and H.GTLD-SERVERS.NET
> are unreachable
> from my view. Preparing an accurate list of all TLD servers
> glued in the
> root zone will take a little longer.

I was taking about root servers at ALL levels, not just the root.

  Perhaps you are using the term "root servers" in a different
  manner than I am used to. For me:

  "Root Server" = a DNS server for the zone "." in the Internet.

  What do you mean by "root servers at ALL levels, not just the root."
  That construction just does not parse.