The system time on the Cisco Nexus 3550-F Fusion (formerly ExaLINK Fusion) can be set manually or synchronized to a number of remote time sources. All time references within the Nexus 3550-F are UTC. Maintaining accurate time is particularly important for packet capture and mirror ports with timestamping enabled. Multiple time synchronization options are available, including PTP (Precision Time Protocol) and NTP (Network Time Protocol).
The time can be set and synchronized via any of the following methods:
- Manually setting the system clock
- GPS sync which can achieve nanosecond level accuracy.
- PTP sync which can achieve microsecond level accuracy.
- NTP sync which can achieve millisecond level accuracy.
- A PPS input can be used to further improve the time sync.
Manual Time Set
In order to set the time manually, use the
time set command. The time format
provided should be YYYY-MM-DD HH:MM:SS, for example:
admin@N3550-F> config time set 2015-12-19 15:37:00 System time updated
GPS Time Sync
The Nexus 3550-F (except early models with serial numbers EXAFSN-A-xxxxx)
includes an onboard GPS receiver.
A GPS antenna can be attached to the appropriate SMA connector on the
front panel allowing the Nexus 3550-F to synchronize the system clock to
highly accurate UTC time. This is enabled using the
timesync gps command, as follows:
admin@N3550-F> configure timesync gps
Using GPS receiver
GPS position and precise UTC time is determined at the point of the GPS antenna, not at the Nexus 3550-F itself. Since the antenna would typically be located some distance from the Nexus 3550-F via a cable, signal propagation delays through the cable cause the time calculated by the receiver to be slightly behind UTC.
Cable delay is a function of the cable type. RG-58 cable for example typically delays the signal 1.54 ns/ft. For 50 feet of cable (15 m) the delay would be 77 nanoseconds.
timesync gps command can optionally take an
antenna-delay parameter to
take antenna cable delay into account, for example:
admin@N3550-F> configure timesync gps antenna-delay 77 Using GPS receiver with 77 ns antenna delay
To check the status of the GPS timesync, use the
show timesync command:
admin@N3550-F> show timesync Sync method : gps GPS fix : yes Location : 41.796754°N 88.24276°W 19.2m Time accuracy : 9 ns Satellites : 12 Last sync time : 2016-04-01 05:41:49
PTP Time Sync
The Precision Time Protocol (PTP) is a protocol used to synchronize clocks throughout a network. On a local area network, it achieves clock accuracy in the sub-microsecond range.
To use PTP, use the following command, optionally specifying the PTP domain to use:
admin@N3550-F> config timesync ptp 0 Using PTP on management interface
To check the status of the PTP timesync, use the
show timesync command:
admin@N3550-F> show timesync Sync method : ptp PTP domain : 0 PTP state : SLAVE Best master : 192.168.220.13 Last offset : 0.000000400 s Clock eth0 : state LOCKED, adev 2308.405000 Clock syst : state LOCKED, adev 61.970000
NTP Time Sync
To synchronize time to an NTP server, use the following command specifying one or more NTP server addresses:
admin@N3550-F> config timesync ntp 192.168.220.13 188.8.131.52 Using NTP on management interface
Status can again be checked using the
show timesync command:
admin@N3550-F> show timesync Sync method : ntp NTP stratum : 2 NTP server : 192.168.220.13 Poll interval : 32 Last offset : 0.044545 s
Disabling Time Sync
To disable timesync, issue the
no timesync command:
admin@N3550-F> config no timesync Time synchronization disabled
PPS Input and Output
A Pulse-Per-Second source can be connected to the Nexus 3550-F to improve the time sync accuracy. Making use of a PPS input is supported for all timesync modes except GPS.
For example, to use a PPS input in conjunction with NTP, use the following command:
admin@N3550-F> config timesync ntp+pps 192.168.220.13 184.108.40.206 rising Using NTP on management interface and PPS
In all cases where a PPS input is used, a time offset can be specified to calibrate out the effect of PPS cable length/delay. For example, to synchronize time using PPS only, and to calibrate out 20ns worth of cable delay, use the following command:
admin@N3550-F> config timesync pps rising cable-delay 20 Using PPS
Setting PPS cable delay requires version 1.10.0 or later
In all cases where a PPS input is used, it's possible to add an internal 50ohm termination resistor to reduce reflections and improve the quality of the PPS signal. In cases where a "daisy chain" of devices are sitting on the PPS network, only the last device in the chain should have PPS termination enabled.
To enable PPS termination, add the
termination keyword to the
timesync command. In the example above where NTP+PPS was used, the command would be:
admin@N3550-F> config timesync ntp+pps 192.168.220.13 220.127.116.11 rising termination Using NTP on management interface and PPS
Enabling PPS termination requires version 1.13.0 or later
The Nexus 3550-F will report whether a valid PPS is being received when the
show timesync command is issued:
admin@N3550-F> show timesync Sync method : ntp NTP stratum : 2 NTP server : 192.168.220.13 Poll interval : 32 Last offset : 0.043768 s PPS edge : rising Termination : enabled Cable delay : 0.000 ns PPS signal : yes
A PPS output can also be generated by the Nexus 3550-F. The accuracy of this PPS edge will be a function of the accuracy of the time reference the Nexus 3550-F is currently using. For example, the PPS output generated from a Nexus 3550-F sync'd via NTP will be much less accurate than that of a Nexus 3550-F sync'd via GPS.
The Nexus 3550-F can generate a PPS output with either a rising or falling edge generated on the second boundary, for example:
admin@N3550-F> configure timesync output pps rising Pulse-per-second time synchronization output enabled
The voltage level of the Nexus 3550-F PPS input and output is 3.3V, however the circuitry is 5V tolerant.
To disable PPS output, issue the
no timesync output pps command:
admin@N3550-F> config no timesync output pps Pulse-per-second time synchronization output disabled
PPS Time Synchronization Fine-tuning
The PPS time sync fine-tuning parameters are only available on the Cisco Nexus 3550-F Fusion HPT and requires version 1.11.0 or later.
When a highly accurate PPS source is used with the Nexus 3550-F HPT, better time synchronization may be achieved by fine-tuning the PPS time sync parameters.
The PPS time synchronization algorithm works by calculating the
frequency and offset correction to be applied to the internal clock.
The frequency and offset are measured over a number of PPS samples, both of
which are configurable by adding the optional
followed by one or two numbers, for example:
admin@N3550-F> config timesync ptp+pps window-size 64 4096
window-size is left out, the default values 64 and 4096 are used.
window-size is present but only one number is provided, the same number
is used for both parameters.
The first parameter is the number of samples (seconds) used to calculate the clock offset. A larger number smooths out jitter in the PPS signal and the sampling thereof. It is recommended to set this value smaller than the second parameter.
The second parameter is the number of samples (seconds) used to calculate the frequency error. A larger number allows a more accurate measurement, but also takes longer to converge and responds slower to changes in frequency.
The smoothed clock offset and the frequency error can be observed using statistics logging. You can use this to guide the selection of the best parameters for your PPS source.
The current system time can be displayed using the
show time command:
admin@N3550-F> show time 2015-12-19 17:12:22
The system uptime can be displayed using the
show uptime command:
admin@N3550-F> show uptime 3 days, 05:44:46.44
This feature requires version 1.10.0 or later
The system timezone can be set with the
config timezone command. Tab completion can be used after typing timezone to list the available countries and timezones defined. For example:
admin@N3550-F> config timezone America/Chicago Timezone set to America/Chicago
This page was last updated on Mar-08-2021.