Apex domains cannot have CNAME records.
Question and response
Question
dig res.cisco.com. NS
Answer
res.cisco.com. 5 NS ns-876.awsdns-45.net.
res.cisco.com. 5 NS ns-1804.awsdns-33.co.uk.
res.cisco.com. 5 NS ns-89.awsdns-11.com.
res.cisco.com. 5 NS ns-1392.awsdns-46.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0