Apex domains cannot have CNAME records.
Question and response
Question
dig customer.level3.net. NS
Answer
customer.level3.net. 2 NS ns15.customer.level3.net.
customer.level3.net. 2 NS ns24.customer.level3.net.
customer.level3.net. 2 NS ns7.customer.level3.net.
customer.level3.net. 2 NS ns9.customer.level3.net.
customer.level3.net. 2 NS ns2.Level3.net.
customer.level3.net. 2 NS ns4.customer.level3.net.
customer.level3.net. 2 NS ns11.customer.level3.net.
customer.level3.net. 2 NS ns12.customer.level3.net.
customer.level3.net. 2 NS ns3.customer.level3.net.
customer.level3.net. 2 NS ns16.customer.level3.net.
customer.level3.net. 2 NS ns21.customer.level3.net.
customer.level3.net. 2 NS ns28.customer.level3.net.
customer.level3.net. 2 NS ns27.customer.level3.net.
customer.level3.net. 2 NS ns17.customer.level3.net.
customer.level3.net. 2 NS ns13.customer.level3.net.
customer.level3.net. 2 NS ns18.customer.level3.net.
customer.level3.net. 2 NS ns1.Level3.net.
customer.level3.net. 2 NS ns22.customer.level3.net.
customer.level3.net. 2 NS ns2.customer.level3.net.
customer.level3.net. 2 NS ns1.customer.level3.net.
customer.level3.net. 2 NS ns19.customer.level3.net.
customer.level3.net. 2 NS ns6.customer.level3.net.
customer.level3.net. 2 NS ns5.customer.level3.net.
customer.level3.net. 2 NS ns8.customer.level3.net.
customer.level3.net. 2 NS ns23.customer.level3.net.
customer.level3.net. 2 NS ns10.customer.level3.net.
customer.level3.net. 2 NS ns25.customer.level3.net.
customer.level3.net. 2 NS ns20.customer.level3.net.
customer.level3.net. 2 NS ns26.customer.level3.net.
customer.level3.net. 2 NS ns14.customer.level3.net.
customer.level3.net. 2 NS ns29.customer.level3.net.
customer.level3.net. 2 NS ns30.customer.level3.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0