mx-res.cisco.com has no CNAME records.
Question and response
Question
dig mx-res.cisco.com. NS
Answer
The response has no answer records.
Authority
cisco.com. 1147 SOA ns1.cisco.com. postmaster.cisco.com. 1178919 7200 1800 864000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0