Our domain previously had multiple sites with writeable domain controllers. These small sites were converted and now only contain a single RODC.
In our DNS I still see glue entries for the writeable domain controllers. The IP addresses are online, but point to a different server that no longer hosts DNS.
Should I update these records to point to the RODCs that now host DNS at the site, or should these records simply be removed as they are no longer writeable servers?
Those glue records should be manually removed because they no longer point to a valid DNS server. If you have zero glue records left for the zone, then you would want to set a new one.
I like this explanation from webdnstools.com:
Glue records are needed to prevent circular references. Circular references exist where the name servers for a domain can't be resolved without resolving the domain they're responsible for.
For example, if the name servers for yourdomain.com are ns1.yourdomain.com and ns2.yourdomain.com, the DNS client would not be able to get to either name server without knowing where yourdomain.com is. But this information is held by those name servers!
This is where glue records are used. A glue record is a hint that is provided by the parent DNS server. In the case of yourdomain.com, the .com GLTD (Global Top Level Domain) servers would provide the glue records. The glue records are simply additional A records that are returned with the DNS response. These additional A records allow the DNS client to locate the name servers.
