ns20.zayo.com has no CNAME records.
Question and response
Question
dig ns20.zayo.com. NS
Answer
The response has no answer records.
Authority
zayo.com. 1800 SOA amber.ns.cloudflare.com. dns.cloudflare.com. 2398325473 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0