Syslog Messages and the debug Command

IOS creates messages, which are called syslog messages, when different events occur and, by default, sends them to the console. The router also generates messages that are treated like syslog messages in response to some troubleshooting tasks that you might perform. The debug command is one of the key diagnostic tools for troubleshooting problems on a Cisco router. It enables monitoring points in the IOS and generates messages that describe what the IOS is doing and seeing. When any debug command option is enabled, the router processes the messages with the same logic as other syslog messages.

The console port always receives syslog messages; however, when you Telnet to the router no syslog messages are seen unless you issue the terminal monitor command. Another alternative for viewing syslog messages is to have the IOS record the syslog messages in a buffer in RAM and then use the show logging command to display the messages. For Telnet users, having the messages buffered using the global config command logging buffered is particularly useful. Finally, the logging synchronous lineconfiguration subcommand can be used for the console and vtys to tell the router to wait until the last command output is displayed before showing any syslog messages onscreen.

Syslog messages also can be sent to another device. Two alternatives exist: sending the messages to a syslog server, and sending the messages as SNMP traps to a management station. The logging host command, where host is the IP address or host name of the syslog server, is used to enable sending messages to the external server. After SNMP is configured, the snmp-server enable traps command tells the IOS to forward traps, including syslog messages.