JN0-106 Operational Monitoring and Maintenance Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  Run 'monitor interface so-0/0/0' and observe for any CRC errors or alarms over a period of time.

  Why this is correct

  This command provides a continuous real-time view of interface statistics and alarms, allowing you to observe transient physical-layer errors such as CRC errors, framing errors, or loss of signal that might occur intermittently. By watching the output over time, you can correlate error bursts with specific conditions (e.g., nearby equipment, cable movement) without disrupting traffic. It directly confirms the health of the physical layer before moving to more invasive testing or hardware replacement.

• ✗

  Perform a loopback test on the interface using 'test interface so-0/0/0'.

  Why it's wrong here

  Loopback testing places the interface into a loopback mode, redirecting transmitted signals back to the receiver to isolate internal hardware faults, but it takes the interface out of service and drops all production traffic on that link. It should only be considered when less intrusive diagnostics have already indicated a specific hardware or cabling fault, not as a first step in troubleshooting intermittent physical errors. Furthermore, on a Junos router, the exact loopback command and behavior may depend on the specific serial PIC hardware, and an internal loopback may not validate external cabling.

• ✗

  Review the system syslog file using 'show log messages' and search for interface down messages.

  Why it's wrong here

  Syslog captures only state transitions and alarms explicitly logged by the routing engine, such as link-up/link-down or high-severity events; it does not record incremental CRC error counters or transient physical-layer anomalies that do not trigger a state change. In many cases, intermittent serial interface errors occur without the interface going down, so there will be no relevant syslog entries, making this method insufficient for detecting the underlying physical degradation. The absence of 'interface down' messages does not imply a healthy physical layer.

• ✗

  Immediately replace the serial interface card to rule out hardware failure.

  Why it's wrong here

  Replacing hardware before collecting empirical evidence is a costly, disruptive, and often ineffective troubleshooting step. The intermittent CRC errors could originate from a faulty cable, improper grounding, electromagnetic interference, or a mismatch in framing or clocking settings rather than from the interface card itself. Only after verifying the physical layer with monitoring, checking configuration, and testing with non-disruptive methods should you consider hardware replacement, and then it's often better to use a swap with a known-good component in a controlled maintenance window.

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