By far, the most popular single IOS command is the show command. The show command has a very large variety of options, and with those options, you can find the status of almost every feature of IOS. Essentially, the show command lists facts about the router's operational status that the router already knows.
Another less popular command is the debug command. The debug command actually tells the router to spend some CPU cycles to do things besides its normal functions, to provide the user with more information about what the router is doing. It requires more router CPU cycles, but it lets you watch what is happening in a router while it is happening.
When you use the debug command, IOS creates messages when different events occur and, by default, sends them to the console. These messages are called syslog messages. If you have used the console of a router for any length of time, you likely have noticed these messages— and when they are frequent, you probably became a little frustrated. You can view these same messages when you have Telnetted to a router by using the terminal monitor command.
Be aware that some debug options create so many messages that the IOS cannot process them all, possibly crashing the IOS. You might want to check the current router CPU utilization with the show process command before issuing any debug command. You also should know that the no debug all command disables all debugs. Before enabling an unfamiliar debug command option, issue a no debug all and then issue the debug that you want to use; then quickly retrieve the no debug all command using the up arrow or Ctrl-p key sequence. If the debug quickly degrades router performance, press Enter immediately, executing the no debug all command, to try to prevent the router from crashing.
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