System Time

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.

The 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 192.168.220.13 121.0.0.41
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 192.168.220.13 121.0.0.41 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

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 192.168.220.13 121.0.0.41 rising termination
Using NTP on management interface and PPS

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

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 window-size parameter followed by one or two numbers, for example:

admin@N3550-F> config timesync ptp+pps window-size 64 4096

If window-size is left out, the default values 64 and 4096 are used. If 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.

Displaying Time

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

Timezones

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.