Symptom:
On Nexus 9000 switch, if there is any Topology Change during the uptime of the switch and if the switch uptime exceeds 90-100 days then the STP timer rolls over (overruns) and STP Topology Change BPDUs are sent every hello interval (ie 2 sec)

Conditions:
Nexus 9000 Switch running for about 90 to 100 days and experienced a TCN during the uptime of the switch.

Active Supervisor Uptime can be found from "show system uptime":

N9K-RTP-ESC# show system uptime
System start time:          Fri Oct 25 09:40:58 2013
System uptime:              236 days, 8 hours, 56 minutes, 59 seconds
Kernel uptime:              110 days, 23 hours, 7 minutes, 49 seconds
Active supervisor uptime:   110 days, 23 hours, 2 minutes, 23 seconds  <<<<<<<<<<<