[c-nsp] Use NTP server for syncing but do not respond to NTP requests

[Andrew Clark]

Take a look at NTP access-groups.  You can control access for each aspect (server, peer, etc).  Details here:

http://www.cisco.com/c/en/us/support/docs/availability/high-availability/19643-ntpm.html

And here:

http://www.team-cymru.org/ReadingRoom/Templates/secure-ntp-template.html

Andrew