Apex domains cannot have CNAME records.
Question and response
Question
dig ipv6.ucsd.edu. NS
Answer
ipv6.ucsd.edu. 5 NS ns-auth3.ucsd.edu.
ipv6.ucsd.edu. 5 NS ns-auth1.ucsd.edu.
ipv6.ucsd.edu. 5 NS ns-auth2.ucsd.edu.
Authority
The response has no authority records.
Additional
ns-auth1.ucsd.edu. 2 A 44.227.162.249
ns-auth2.ucsd.edu. 2 A 132.239.252.172
ns-auth3.ucsd.edu. 2 A 132.239.252.186
ns-auth2.ucsd.edu. 2 AAAA 2607:f720:100:202:0:0:0:174
ns-auth3.ucsd.edu. 2 AAAA 2607:f720:f00:4446:0:0:0:186
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0