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Terminal monitor

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

terminal monitor is a Cisco IOS privileged EXEC command that enables real-time monitoring of Syslog messages for the current VTY session.

terminal monitor enables real-time Syslog messages for the current Telnet or SSH session on a Cisco IOS device. Although VTY logging is enabled by default, messages remain hidden until this privileged EXEC command is issued. section 7.1.1

Use the command and its session-specific counterpart as follows:

  • Enter terminal monitor after every Telnet or SSH connection; disconnecting and reconnecting requires the command again. section 7.1.1
  • Each simultaneous VTY user must issue terminal monitor independently; enabling it for one user does not affect others. section 7.1.1
  • Stop messages for the current session with terminal no monitor, not no terminal monitor. section 7.1.1

Watching an interface event over SSH

After issuing terminal monitor, reenable an interface with no shutdown. The session displays link and line-protocol Syslog messages; without terminal monitor, the same event produces no visible VTY output. section 7.1.1

SW1(config)# interface g0/1
SW1(config-if)# shutdown                                                            #1
SW1(config-if)# do terminal monitor                                                 #2
SW1(config-if)# no shutdown                                                         #3
Sep 18 02:17:46.867: %LINK-3-UPDOWN: Interface FastEthernet0/3, changed state to up #4 
Sep 18 02:17:47.874: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/3, changed state to up                                                                 #4
#1 No Syslog messages are shown after disabling G0/1.
#2 Enables real-time logging for the session
#3 Reenables the interface
#4 Syslog messages are displayed.

Follow the shutdown, session-monitoring, and no shutdown sequence with annotated Syslog output. section 7.1.1

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes
Explains defaults, session scope, repeated use, disablement syntax, and an interface-event demonstration.
Examples
Uses interface shutdown and no shutdown commands to generate and observe link-state Syslog messages.
What it assumes
Assumes Telnet or SSH access and privileged EXEC access while clarifying that VTY logging itself is enabled by default.
What you can do after
Operate real-time Syslog monitoring per VTY session and disable it with the correct command.
How it connects
Connects Syslog with VTY lines, Telnet, SSH, logging monitor, and privileged EXEC mode.
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