It looks like there hasn't been much of an impact in the past?
http://www.cisco.com/c/en/us/about/supplier-sustainability/leap-second.html#~Response,
"We assessed the related records for all three periods, spanning all Cisco platforms and all Cisco customers, and found a limited number of defects and service requests. Although one of these customers required a system reboot to recover, most experienced synchronization alarms, which were easily and quickly resolved with support from the Cisco Technical Assistance Center (TAC). As a result, we do not believe that the previous leap-second adjustments represented any material events for our products or customers."
You will see there are some COP files available for the issue if you want to apply them.
