When configuring a Cisco router to function as an NTP authoritative server, the 'server' mode must be activated. This mode allows the router to provide time synchronization to NTP clients within the network, even if it is not synchronized to an external time source. The 'server' mode ensures that the router's time is considered authoritative and can be distributed to other devices. References: Cisco's configuration guides provide instructions on setting up a Cisco router as an authoritative NTP server, including the necessary commands and configurations