IP services: NTP and PTP, NAT and PAT, HSRP and VRRP

Accurate time underpins logging, certificates, authentication and troubleshooting. NTP (Network Time Protocol, UDP port 123) synchronizes device clocks to a reference. Its hierarchy is measured in stratum: stratum 0 is a reference clock such as GPS or an atomic clock, a server directly attached to it is stratum 1, and each hop adds one. Lower stratum is closer to the source; 16 means unsynchronized. On IOS, ntp server 192.0.2.10 makes the device a client, and ntp master makes it an authoritative source using its own clock. Protect NTP with authentication (ntp authenticate, ntp authentication-key, ntp trusted-key) and verify with show ntp status and show ntp associations. NTP typically achieves millisecond-level accuracy. PTP (Precision Time Protocol, IEEE 1588) achieves much higher precision, down to sub-microsecond, by using hardware timestamping in network interfaces. It elects a grandmaster clock with the best master clock algorithm, and switches can act as boundary clocks or transparent clocks that correct for their own delay. PTP matters for industrial automation, broadcast audio and video, and financial trading.