expiring secondary zones and parent delegation on Bind9

[Jan Gyselinck]

The following situation exists:

parent zone x.com (running on company-ns-1 and company-ns-2),
delegates child.x.com to

company-ns-1
company-ns-2
customer-ns-1
customer-ns-2

child zone child.x.com, runs on company-ns-1 and company-ns-2, slaving
off customer-ns-1.

Now child.x.com expires on company-ns-1 and company-ns-2 after not being
able to reach customer-ns-1 for some time.  No worries, since the
parent also delegates to the customers' nameservers directly.  But,
now company-ns-1 and company-ns-2 do not return the 4 ns records
for child.x.com which are defined in x.com (it only returns SERVFAIL),
hereby breaking child.x.com completely (delegation to customer-ns-1
and customer-ns-2 becomes effectively 'invisible').

As far as I can tell this didn't happen with bind 8.  We've recently
upgraded to bind 9 and this issue suddenly pops up.  Should I consider
this as normal behaviour?

Regards


Jan Gyselinck