IOS Syslog facilities

Logging and debugging messages are the cornerstone of troubleshooting.

There are 4 possible destinations for Logging:
1.Console
2.Monitor
3.Buffer
4.Host
+plus SNMP

R1(config)#do sh logging
Syslog logging: enabled (9 messages dropped, 1 messages rate-limited, 0 flushes, 0 overruns, xml disabled)
Console logging: level debugging, 33 messages logged, xml disabled
Monitor logging: level debugging, 0 messages logged, xml disabled
Buffer logging: disabled, xml disabled
Logging Exception size (8192 bytes)
Count and timestamp logging messages: disabled
Trap logging: level informational, 38 message lines logged

Descriptions for the fields in the output above.

When troubleshooting in High CPU utilization situations, and you have to use debugging, make sure to disable or "level-limit" the console and monitor logging facilities. Instead use the buffered logging facility to record the debug output and view it with "show logging". This will allow the CPU to process the log messages in a high utilization environment.

To use a syslog server and set the log-level:

R1(config)# logging host
R1(config)# logging trap ?
<0-7> Logging severity level
alerts Immediate action needed (severity=1)
critical Critical conditions (severity=2)
debugging Debugging messages (severity=7)
emergencies System is unusable (severity=0)
errors Error conditions (severity=3)
informational Informational messages (severity=6)
notifications Normal but significant conditions (severity=5)
warnings Warning conditions (severity=4)

Reference:
Troubleshooting and Fault Management Commands @ Cisco
Troubleshooting High CPU Utilization on Cisco Routers @ Cisco