3.3 IP Services
3.3.a Interpret network time protocol configurations such as NTP and PTP
R1(config)# ntp master [#] = tell R1 to become an NTP server of # stratum
R2(config)# ntp server [ip address] = tell R2 to become an NTP client of [server ip address]
R2(config)# ntp server [ip address] source [loopback int] = use the loopback’s IP as source for NTP packets, because the loopback address is stable and doesn’t change
R1# show ntp status = show very verbose info about the NTP process on the device
R2# show ntp associations = preferred way to check, shows NTP clients and servers
If multiple NTP servers are setup on a device, device will prefer the lowest stratum
R1(config)# ntp peer [R2], R2(config)# ntp peer [R1]
NTP peering = two devices can set to peer each other, the device will take the time from the peer that is most accurate
- Stratum — generally, a lower stratum is preferred.
- Reachability — the source must actually be responding reliably.
- Offset — how far the device’s clock differs from that source.
- Delay — network delay to the source.
- Dispersion/jitter — how stable/accurate the source appears over time.
PTP (https://standards.ieee.org/ieee/1588/4355/), when a few miliseconds of drift is not good enough
- Grandfather clock is designated, best clock in the domain
- All other devices synchronize to it
- PTP accounts for processing delay, travel time, propagation delay (time it takes to put packets on the link)
- Devices exchange timing messages to calculate this delay as the PTP packet passes through them, lets them know downstream and downstream can correct their time to compensate
Grandmaster = best clock in domain
Ordinary clock = a PTP endpoint, i.e. a device syncing their own clock to max accuracy for operations
Boundary clock = sync to grandmaster, and serves downstream devices
Transparent clock = as PTP packets inevitbaly pass through intermediate devices, transparent clocks perform PTP calculations and send downstream to boundary/ordinary clocks
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