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Trusted key

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

ntp trusted-key is a Cisco IOS configuration command that marks an NTP authentication key as trusted for use.

The Cisco IOS ntp trusted-key command authorizes a previously created NTP authentication key for use. Creating an authentication key alone is insufficient; an untrusted key cannot authenticate NTP messages. section 2.3.3

Configure trusted-key authentication consistently on both NTP devices:

  • Create the same key number and password with ntp authentication-key key-number md5 key. section 2.3.3
  • Mark that key as usable with ntp trusted-key key-number. section 2.3.3
  • On a client, reference it with ntp server ip-address key key-number; configure the equivalent ntp peer command on both peers. section 2.4

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes
Explains key creation, trust activation, matching requirements, and server or peer association, but does not detail verification commands.
Examples
Uses Cisco IOS configurations for R1 and R2 with key number 42 and password AcingTheCCNA!.
What it assumes
Assumes basic NTP client, server, and peer configuration while defining authentication keys and trust activation explicitly.
What you can do after
Operate/configure Cisco IOS NTP authentication so a client accepts time only from a server using matching trusted credentials.
How it connects
Connects NTP authentication with NTP client/server mode and symmetric active NTP peers.
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Related concepts

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