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Cisco Bug: CSCsz32078 - clock set to 1941 after nDSC rack oir

Last Modified

May 17, 2018

Products (1)

  • Cisco Carrier Routing System

Known Affected Releases

3.6.3.BASE

Description (partial)

Symptom:

Clock value gets set to invalid values and NTP is not able to correct it.


Condition:

The fault lays in the way NTP calculates the offset (This is a well known limitation in the version of NTP in XR 3.7 and earlier releases).
The existing logic only allowed a offset of +/- 34 years to be recognized succesfully, anything over that limit resulted in an overflow and this was what seen on the MC CRS testbed.

The only permitted operation on raw timestamps is subtraction, where the result is a signed quantity spanning from 68 years in the past to 68 years in the future. To avoid loss of precision, these calculations are done using 64-bit integer arithmetic. However, the offset and delay calculations are sums and differences of these first-order differences, which if done using 64-bit integer arithmetic, would be valid over only half that span. Since the typical first-order differences are usually very small, they are converted to 64-bit doubles and all remaining calculations done in floating-point arithmetic. This preserves the accuracy while retaining the 68-year span.
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