subdomain forwarding on a domain-authoritative dns

[Philippe.Simonet at swisscom.com]

Hello ISC users

in a special environment, I have to forward 'sub.domain.com', on a dns where I'm 
master for 'domain.com'. all subsequent subdomains of 'sub.domain.com' must
be reachede through this forwarder. I cannot use iteration here because the
dns authoritative for e.g. 'sub1.sub.comain.com' are not reachable from this dns.

zone domain.com {
	type master;
	file "db.domain.com";
}
zone sub.domain.com {
	type forward;
	forwarders { 1.2.3.4; };
}

all this is is working only if I define a NS for sub.domain.com in 'domain.com' : 
----
sub.domain.com	IN NS	subns.domain.com
subns.domain.com  IN A  1.2.3.4
----
I also observed that this NS is not used by bind, because a dummy address is enough
to do the forward correctly. this NS has only to be present and the forwarding works.


my question is : is this a bind 'feature' or is this a bad idea to use that ?
I have to ensure that this behaviour will be the same in the next bind releases. 

thanks and regards

Philippe Simonet