conflicting subdomain delegation

[Frank Liu]

Hi,

Is there a RFC determining which nameserver to use if there is a
conflicting subdomain delegation?

eg:
In the zone of a.com, there are two NS delegations:

b.a.com NS host1
c.b.a.com NS host2

On host1 in zone b.a.com, there is
c.b.a.com NS host3

As you can see, there is a conflicting delegation for c.b.a.com. If I look
a name d.c.b.a.com, will the nameserver host2 or host3 be used?
dig +trace seems to go to host2, but bind9 as a resolver goes to host3.
(the test was done on a centos7).

Any ideas?
Thanks!
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