I have found 2 ways :
Method 1
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[no] ntp authenticate
[no] ntp authentication-key number md5 value
ntp server ip_address [key number] source if_name [prefer]
no ntp server ip_address
[no] ntp trusted-key number
clear ntp
show ntp
show ntp associations [detail]
show ntp status
Method 2
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!Find a external NTP source a.b.c.d
ntp server source outside
ntp server source outside
replace w/ the IP address of the NTP source.
comments - opinions ?