Apex domains cannot have CNAME records.
Question and response
Question
dig softlayer.net. NS
Answer
softlayer.net. 5 NS ns1.networklayer.com.
softlayer.net. 5 NS ns1.softlayer.net.
softlayer.net. 5 NS ns2.networklayer.com.
softlayer.net. 5 NS ns2.softlayer.net.
Authority
The response has no authority records.
Additional
ns1.softlayer.net. 5 A 67.228.254.24
ns1.networklayer.com. 5 A 67.228.254.22
ns2.softlayer.net. 5 A 67.228.255.25
ns2.networklayer.com. 5 A 67.228.255.22
ns1.softlayer.net. 5 AAAA 2607:f0d0:0:15:1:0:0:1
ns1.networklayer.com. 5 AAAA 2607:f0d0:0:15:10:0:0:1
ns2.softlayer.net. 5 AAAA 2607:f0d0:0:15:2:0:0:1
ns2.networklayer.com. 5 AAAA 2607:f0d0:0:15:20:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0