Which NTP command configures the local device as an NTP reference clock source?

'ntp peer' configures a symmetric active association for mutual time synchronization between two devices, not to designate a device as a reference source.

'ntp broadcast' causes an interface to send NTP time updates as broadcast packets to clients, but does not make the device an authoritative reference clock.

The 'ntp master [stratum]' command configures the router or switch to act as an authoritative NTP reference clock, allowing it to serve time to NTP clients even without an upstream time source. It sets a local stratum value (default 8) so other devices treat it as a valid time authority. Without this command, the device will not provide time to peers if it cannot reach an external NTP source.

'ntp server' points the local device to an external upstream NTP server to synchronize from, which is the opposite of becoming a source.