I don't understand this. Everywhere I read that glue records are only required to prevent circular dependencies, for example I want to set ns1.example.com and ns2.example.com nameservers for the example.com domain name.
But according to my experience it is not true. I want to create my own nameservers such as ns1.example.com and ns2.example.com. Example.com is my domain name, and I already have the 2 IPs for my nameservers and the nameservers work. I just need to create hostnames for my nameserver IPs.
So I added 2 A records for my example.com domain name using the DNS zone file editor (ns1.example.com. 300 IN A [nameserverip1] and ns2.example.com. 300 IN A [nameserverip2]). Example.com uses an external DNS service, it will never use the ns1.example.com and ns2.example.com nameservers, so in theory glue records are not required.
However this is what I experienced: there is example.net and I want to use the ns1.example.com and ns2.example.com nameservers for example.net, and guess what, it doesn't work. Because when I try to enter these nameservers, I get the error: Entity reference not found [host ns1.example.com not found.]
But after I open the control panel of my example.com domain name registrar and register the ns1.example.com and ns2.example.com nameservers (I create glue records), then it will work.
But I want to avoid setting the glue records, because I cannot control the TTL of the glue records, so if the IPs will change, I may experience downtime.
Why isn't it enough to only add the A records for ns1.example.com and ns2.example.com? Why must I register them at my domain name registrar?
Update: according to my experience, it is only an issue with .com, .net and .org domain names. Is it true that these TLDs require glue records even if there is no circular dependency? Because for example when I use the ns1.example.hu and ns2.example.hu nameservers, they work only with A records and no glue records. Seems like the .hu root servers behave differently than the .com root servers?