Consider the following from a fictive DNS.
a. NS c.b.a. NS f.e. b.a. NS d.b.a. d.b.a A 1.2.3.4
If these records are an excerpt of one DNS zone file, it must be the one carrying the DNS data for zone 'a', otherwise there were a mistake because you cannot first delegate zone 'a' and thereafter 'b.a' which is a subdomain of 'a'. So the first two NS-RRs are the authoritative information for zone 'a'. You do not need (and want) glue records here, inside the zone. Zone 'b.a' is delegated out of zone 'a' and server 'd.b.a' resides in zone 'b.a', so the glue record - as shown in your example - should be present. However, when delegating 'a' the glue RR for 'c.b.a' is necessary in the delegating zone. -Peter PS: Here's another one, from zone 'foo': bar.foo. NS a.rab.foo. NS b.rab.foo. rab.foo. NS c.bar.foo. NS d.bar.foo.