Someone or something caused your router to reboot. You want to log configuration changes to rule that out without using TACACS. Here’s what it looks like.
router(config)# archive
router(config-archive)# log config
router(config-archive-log-cfg)# logging enable
router(config-archive-log-cfg)# logging size 1000
router(config-archive-log-cfg)# notify syslog
router(config-archive-log-cfg)#
To display configuration log entries by record numbers starting with the first recorded command
show archive log config 1
To display all configuration log files as they would appear in a configuration file rather than in a tabular format
show archive log config all provisioning
To view statistics
show archive log config statistics
router# show archive log config statistics
Config Log Session Info:
Number of sessions being tracked: 1
Memory being held: 3909 bytes
Total memory allocated for session tracking: 187657 bytes
Total memory freed from session tracking: 183748 bytes
Config Log log-queue Info:
Number of entries in the log-queue: 63
Memory being held by the log-queue: 16356 bytes
Total memory allocated for log entries: 16356 bytes
Total memory freed from log entries: 0 bytes
For more information: http://www.cisco.com/c/en/us/td/docs/ios/fundamentals/configuration/guide/15_1s/cf_15_1s_book/cf_config-logger.html