Apex domains cannot have CNAME records.
Question and response
Question
dig globalextgw.gva.es. NS
Answer
globalextgw.gva.es. 5 NS nsp1vpxgw.globalextgw.gva.es.
globalextgw.gva.es. 5 NS nsp2vpxgw.globalextgw.gva.es.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0