In order to reach the authoritative dns servers for a particular domain, a dns cache must first obtain the names of the assigned name servers. Then it must obtain the related ip addresses.
The address records name servers registered at the parent domain are known as glue records. The glue record contents are inserted as additional information in the answer to a query that contains the name server records in question.
In those cases where a domain uses name servers which are in another domain, the administrator associates existing name servers for the domain but does not register name servers. Since these name servers are already registered, associating a name server does not involve glue records.
It is extremely bad practice to attempt to alias an existing name server by creating zone records which point to a name server which is already registered under a different name. This practice invites problems in that the glue address records for the registered name servers are under the control of a separate entity and may change at any time without notice. In that event, name resolution will fail for the domain.