VersaPNT can be configured to receive time from one or more available NTP Servers (VersaPNTs or different models). This allows for NTP Servers on a timing network to be configured as potential (fallback) input time references for VersaPNT System Time synchronization. In the event that a current reference becomes unavailable, VersaPNT can fallback to the other NTP Servers available on the network.
A group of NTP servers at the same Stratum level (Stratum 1 time servers, for example) are considered as NTP Peers to each other.
NTP Servers at a higher Stratum level, on the other hand, are called NTP Servers (Note that Internet Time Servers should be configured as NTP Servers and not as NTP Peers).
Note: IMPORTANT: In order for other NTP servers to be a valid reference, you must enable “NTP” in the Reference Priority table (see Configuring Input Reference Priorities).
For mutual fallback purposes, it is recommended to use one or more NTP Peers. Each peer is normally configured to operate from one or more time sources including reference clocks or other higher stratum servers. If a peer loses all reference clocks or fails, the other peers continue to provide time to other clients on the network.
If VersaPNT is configured to obtain time from other NTP Servers at the same Stratum level (i.e., NTP Peers) but is currently using a different input reference as its selected reference, VersaPNT will report to the network (via the NTP time stamps) that it is a Stratum 1 time server. Should, however, all input references except the other NTP server(s) become unavailable, VersaPNT will then drop to a Stratum 2 time server (with System Time being derived from the NTP time packets being received from the other NTP Peers.
If VersaPNT is configured to obtain time from another NTP Server at a higher Stratum level (i.e., NTP Servers), and it is using that NTP Server as its selected reference, VersaPNT will report to the network (via the NTP time stamps) that it is one less Stratum than its selected reference NTP Server.
If VersaPNT is configured to receive time from one or more Stratum 1 NTP Servers, with no other higher priority input references available, VersaPNT will report to the network that it is a Stratum 2 Server.
In order for VersaPNT to use other NTP servers as a valid time reference to synchronize the System Time, the input Reference Priority Setup table must be configured to allow NTP as an available reference. For more information on the input Reference Priority table, refer to Configuring Input Reference Priorities.
If VersaPNT is synchronized to another NTP Server or reference, and that server or reference subsequently loses sync or becomes unavailable (with no other higher priority input references being present and valid), VersaPNT will then go into the Holdover mode. It will remain in Holdover mode until any enabled and valid input reference becomes available again, or until the Holdover period expires, whichever occurs first.
During Holdover mode, NTP will remain at the same Stratum level it was before entering the Holdover mode and can continue to be the reference to the network. However, if no input reference becomes available before the Holdover period expires, Time Sync will be lost and shortly thereafter, NTP will report to the network that it is now at Stratum 15. A status of Stratum 15 will cause the network to ignore VersaPNT as an NTP time reference.
For more information about Holdover, see Holdover Mode.