Meist ist es durch ein Konfigurationsbeispiel einfacher zu verstehen, wie die NTP mit Hilfe einer ACL abgesichert werden kann.

Konfigurationsbeispiel:

! Core configuration
ntp update-calendar
! update hardware clock (certain hardware only, i.e. 6509s)
ntp server 192.168.0.1
! a time server you sync with
ntp peer 192.168.0.2
! a time server you sync with and allow to sync to you
ntp source Loopback0
! we recommend using a loopback interface for sending NTP messages if possible
!
! NTP access control
ntp access-group query-only 1
! deny all NTP control queries
ntp access-group serve 1
! deny all NTP time and control queries by default
ntp access-group peer 10
! permit time sync to configured peer(s)/server(s) only
ntp access-group serve-only 20
! permit NTP time sync requests from a select set of clients
!
! access control lists
access-list 1 remark utility ACL to block everything
access-list 1 deny any
!
access-list 10 remark NTP peers-servers sync to
access-list 10 permit 192.168.0.1
access-list 10 deny any
!
access-list 20 remark Hosts-Networks allow to get
access-list 20 permit 192.168.0.0 0.0.0.255
access-list 20 deny any