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Cisco Bug: CSCvi67983 - Spikes in PTP mean path delay value can be observed intermittently

Last Modified

Sep 05, 2018

Products (1)

  • Cisco Nexus 3000 Series Switches

Known Affected Releases

6.0(2)A8(6)

Description (partial)

Symptom:
Periodically Mean path delay value reported as very high (around 0.5 sec) on slave PTP switch

`show system internal ptp info calculations`
...
134) Event:(null), length:8, at 276528 usecs after Fri Feb  2 00:05:38 2018
131) Event:(null), length:8, at 547939 usecs after Fri Feb  2 00:05:40 2018
Mean path delay 54

132) Event:(null), length:48, at 76923 usecs after Fri Feb  2 00:05:40 2018
slave_if 1a000000, master_ts 1517529977 s 37155840 ns,slave_ts 1517529977 s 37155943 ns, ts_err 0 -50


133) Event:(null), length:8, at 788014 usecs after Fri Feb  2 00:05:39 2018
Mean path delay 53

134) Event:(null), length:8, at 276528 usecs after Fri Feb  2 00:05:38 2018
Mean path delay 505001966

135) Event:(null), length:48, at 3693 usecs after Fri Feb  2 00:05:37 2018
slave_if 1a000000, master_ts 1517529973 s 997105950 ns,slave_ts 1517529973 s 997105980 ns, ts_err 0 22

It results in high offset from master in "show ptp clock" - until next mean path delay calculation.

Conditions:
Issue was observed on PTP Slave device in the following scenario, 
N3500 (PTP slave) === N3500 (PTP master) === GMC

This issue was observed when workload on CPU of Master PTP switch is high (heavy SNMP polling, e.t.c). On this scenario some of PTP followup packets can be missed.
Miss of followup packets itself is not a problem (it just causes PTP correction will not occur when PTP followup is missed).
In some conditions when followup packets are missed (exact condition not known yet) it causes PTP slave to take incorrect t1/t2 values for mean path delay calculation, e.g. takes t1 and t2 from packets with not-matching sequence ID.
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