– The hardware clock has its own battery. When router shuts down, hardware calendar still works.
– When we reboot router, the software clock synchronizes with hardware clock, even if we already set the software clock.
– Once the software clock is user-defined, we can synchronize the hardware clock with it, with “clock update-calendar”.
– with “show clock” and “show clock detail”, if an asterisk is displayed then the time server is not authoritative which means that it is not reliable.
– notion of stata. Each time server has a stratum. Server with stratum 0 has more precise time than server with stratum 1, etc.
– Server with stratum 1 feeds server with stratum2, which feeds server with stratum3, etc.
– NTP synchronises software clock only, by default. We can instruct NTP to synchronize hardware clock too with “ntp update-calendar”.