What is the debug command and how should it be used?

debug displays OS process/protocol/event messages in real time (privileged EXEC); use it narrowly for specific problems because it consumes heavy resources, and turn it off with undebug all.

The debug Command:

The IOS debug command allows the administrator to display OS process, protocol, mechanism, and event messages in real-time for analysis.

Key points:

• All debug commands are entered in privileged EXEC mode
• Cisco IOS allows narrowing debug output to include only the relevant feature or subfeature
• Use debug commands only to troubleshoot specific problems

Debug commands:

# List all debugging options
debug ?
# Debug ICMP messages
debug ip icmp
# Turn off specific debug
undebug ip icmp
# Turn off ALL active debug commands
undebug all

terminal monitor command:

• debug output is not automatically displayed on remote connections (like SSH)
• Use terminal monitor in privileged EXEC mode to display log messages on a remote terminal
• Use terminal no monitor to stop logging messages on a terminal

Warning: Be cautious with debug commands - they may generate a substantial amount of output and use a large portion of system resources. The router could get so busy displaying debug messages that it cannot perform its network functions.