NTP uses UDP port 123 to communicate with its servers and peers. If the Cisco ASA has IP reachability to the NTP server and is not filtering any traffic, then the most likely cause of the unsynchronized state and the stratum of 16 is that the NTP server is not working properly or is not configured to provide NTP service. A stratum of 16 means that the NTP server is unreachable or is not considered a viable candidate1. The value.INIT. in the refid column indicates that NTP is initializing, and the server has not yet been reached1. To resolve this issue, the network engineer should verify the status and configuration of the NTP server, and use a different server if needed. Alternatively, the network engineer can use the ntp server command with the prefer keyword to specify a preferred NTP server, or use the ntp update-calendar command to force a resynchronization of NTP2. References:

1: NTP server is unsynchronized or unreachable, a Wireshark perspective.

2: Implementing and Operating Cisco Security Core Technologies (SCOR) v1.0, Module 1: Network Security, Lesson 1.3: Configuring Network Time Protocol.