You Telnet into a remote Cisco router and enter the command debug ip rip, but no debug output appears in your session. What is the most likely reason for this, and what should you do?

Introduction / Context:
Cisco IOS debug commands are powerful tools for troubleshooting routing protocols such as Routing Information Protocol (RIP). However, many network engineers are surprised when they enable a debug command during a Telnet or Secure Shell session and see no output. This question focuses on how Cisco IOS handles debug output and which additional command is required to display that output on a virtual terminal line. Understanding this behaviour is vital for real time troubleshooting in production networks.

Given Data / Assumptions:

• Access to a Cisco router via Telnet, not through the console port.
• The command debug ip rip has been entered successfully in privileged EXEC mode.
• No debug output is visible in the Telnet session even though RIP is assumed to be operating.
• The goal is to understand why and to identify the correct corrective command.

Concept / Approach:
By default, Cisco IOS sends debug output to the console line only. Virtual terminal lines such as Telnet and Secure Shell do not receive debug output unless they are explicitly enabled to do so. The command that instructs the router to display logging and debug messages on the current virtual terminal session is terminal monitor. Without this command, the debug messages are still generated and can be seen on the console, but they do not appear in the Telnet session, which leads many administrators to think the debug is not working.

Step-by-Step Solution:
1. Confirm that the user is connected through a Telnet session, not via the console port. 2. Enter privileged EXEC mode and issue the command debug ip rip. 3. Observe that no output appears in the Telnet session, even though the router is exchanging RIP updates. 4. Recall that default debug output is directed to the console line only. 5. On the same Telnet session, enter the command terminal monitor. 6. After enabling terminal monitor, debug ip rip output will start to appear in the Telnet session as RIP packets are processed.

Verification / Alternative check:
As an alternative check, you can connect to the router through the console port and enable debug ip rip there. You will see immediate debug messages on the console without needing terminal monitor. This confirms that the debug feature itself is working. Another verification method is to use the show logging command to review logging configuration and ensure that logging to the monitor is enabled by default. In almost all standard configurations, using terminal monitor in the Telnet session is sufficient to view debug messages.

Why Other Options Are Wrong:
You must enable RIP routing with the router rip command before using debug ip rip: RIP is already assumed to be running, and even if it were not, the debug command would still show some messages such as error or status information. Lack of output in a Telnet session is due to output destination, not protocol activation.
IP addressing on the network is incorrect so no debug messages are generated: Incorrect addressing could reduce the number of updates, but usually some debug messages still appear. Moreover, the question specifically focuses on Telnet behaviour.
You must first run show ip rip for debug output to appear: Show commands and debug commands are independent. Show commands do not enable or unlock debug output.
Debug ip rip can only be executed from privileged EXEC mode on the console: Debug commands can be executed from privileged EXEC mode on any line, including virtual terminal lines, as long as the user has the correct privilege level.

Common Pitfalls:
Many learners forget that terminal monitor is required for Telnet and Secure Shell sessions, which leads to unnecessary troubleshooting of routing protocols instead of the session. Some administrators also leave debug commands running for too long, which can overload the router CPU. Best practice is to use debug selectively, view the needed output, and then disable debugging with undebug all or no debug all to maintain performance. Always remember where debug output is being directed: console, terminal monitor, or logging buffer.

Final Answer:
You must use the terminal monitor command so that debug output is sent to the Telnet session