When you got an outage, you want to see a log first on your device. Well, if you have NMS or SNMP tool, you prefer to see the log. It would be just me, but I like to see a log data on device which was directly involved with the outage. One of reason is that I don’t trust message from somebody is few step away from the event. Actually, I did see few cases that monitoring system had issue itself or issue on route path to reach the monitoring system.

By the say, even if your device didn’t setup log option, you will be frustrated in order to figure out what was happening.

Here is simple way to enable log function on your device(Cisco in this case)

Router# config t
Router(config)# service timestamps debug datetime localtime msec
Router(config)# service timestamps log datetime localtime msec
Router(config)# logging buffered
Router(config)# ^Z


Default logging buffer size will be 4096 which is okay, but if you have a lot of links and heavy user then increase the value.
Below example show logging buffered size to 8192

Router(config)# logging buffered 8192
Router(config)# ^Z
Source: http://cisconet.com/vendor/cisco/214-cisco-how-to-enable-log-and-logging-buffer.html