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The debug and show Commands

By far, the single most popular Cisco IOS command is the show command. The show
command has a large variety of options, and with those options, you can find the status
of almost every feature of Cisco IOS. Essentially, the show command lists the currently
known facts about the switch’s operational status. The only work the switch does in reaction
to show commands is to find the current status and list the information in messages
sent to the user.

For example, consider the output from the show mac address-table dynamic command
listed in Example 6-3. This show command, issued from user mode, lists the table the switch
uses to make forwarding decisions. A switch’s MAC address table basically lists the data a
switch uses to do its primary job.

Example 6-3 Nondefault Basic Configuration
Free CISCO CCNA Routing and Switching ICND1 Study Guide

The debug command also tells the user details about the operation of the switch. However,
while the show command lists status information at one instant of time—more like a photograph—
the debug command acts more like a live video camera feed. Once you issue a
debug command, IOS remembers, issuing messages that any switch user can choose to see.
The console sees these messages by default. Most of the commands used throughout this
book to verify operation of switches and routers are show commands.

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