Reference: https://www.cisco.com/c/en/us/support/docs/ip/network-time-protocol-ntp/108076-ntp-troubleshoot.html Network Time Protocol (NTP) uses the User Datagram Protocol (UDP) to communicate. Specifically, NTP messages are transmitted over UDP port 123. Therefore, to allow NTP services to work properly, UDP must be allowed on the firewall. Reference: Cisco DevNet Associate documentation on network protocols, including the details of NTP and its use of UDP.