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There are periodic leap second events which can add or delete a second to global time. The leap second event can be propagated via Precision Time Protocol (PTP) if configured. When the leap second update occurs and the device is configured to use PTP as a Boundary Clock (BC) then an incorrect Coordinated Universal Time (UTC) offset and PTP Leap Indicator could propagate incorrect time downstream. This could cause the PTP network time to be off as much as thirty five seconds.
The leap second update is propagated from the configured PTP Grandmaster clock. A Cisco device running the PTP protocol would have the "feature ptp" command in the running configuration.
For this problem of the incorrect PTP UTC offset and Leap Second update flag for Cisco devices configured as a PTP boundary clock the following workaround can be used: 1. Increase the Time To Live (TTL) on the IPV4 multicast PTP frames from the PTP Grand Master clock from the default of one (1) to something higher than the number of multicast hops the PTP packets would have to traverse in the network to reach the Cisco device. 2. Disable PTP on the affected cisco devices configured as PTP boundary clocks. 3. If the now disabled Cisco devices configured as a PTP boundary clock supports Internet Group Management Protocol (IGMP) snooping then nothing additional is required. If the device does not support IGMP snooping then static multicast Content Addressable Memory (CAM) entries would need to be created for the devices downstream which need the PTP frames from the PTP Grand Master clock. If this workaround cannot be implemented than an upgrade is recommended.
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