CCNA Network Infrastructure and Connectivity 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.

• ✓

  Identify the interface with the link-down condition using the 'show interfaces' command and note any input/output errors.

  Why this is correct

  This is the correct first troubleshooting step because 'show interfaces' isolates the specific port that is down and displays the current line protocol state. The output also provides input/output error counters, such as CRC, runts, and giants, which help distinguish between a physical-layer issue (no light, SFP failure) and a data-link-layer misconfiguration (like duplex mismatch). Recording these errors before reseating or replacing the SFP gives a baseline to confirm whether the problem is resolved.

• ✓

  Verify the physical connection matches the SFP type (e.g., fiber patch cord for fiber SFP, copper cable for copper SFP) and the connector is secure.

  Why this is correct

  Correct: This step verifies the media type matches the SFP transceiver's specification, e.g., a 1000BASE-SX SFP requires a multimode fiber patch cord, while a 1000BASE-T SFP uses an RJ-45 copper cable. Using the wrong patch cord, such as single-mode fiber with a multimode-only SFP, or a loose/bent connector, will prevent the link from coming up even if the SFP itself is working. A visual and physical inspection also catches dirty or damaged connectors, which are common causes of optical link failure.

• ✓

  Confirm the SFP transceiver is properly seated and recognized by the switch using 'show inventory' or 'show interface transceiver.'

  Why this is correct

  Correct: This step checks that the SFP is properly inserted into the switch's slot and that the switch's IOS recognizes the transceiver. If the SFP is not fully seated or is unsupported/counterfeit, 'show inventory' will not list it as a valid transceiver, and 'show interface transceiver' will show 'Unknown' or no module data. A loosely seated SFP can pass visual inspection but fail electrically, so verifying recognition ensures the switch can communicate with the SFP's EEPROM and power the optics.

• ✓

  Examine the SFP’s health and optical power levels with 'show interfaces transceiver detail' to check TX/RX power, temperature, and voltage.

  Why this is correct

  Correct: The 'show interfaces transceiver detail' command leverages the SFP's Digital Diagnostic Monitoring (DDM) to report real-time optical TX/RX power, temperature, and voltage. Comparing these values to the manufacturer's thresholds identifies whether the receive power is too low (e.g., below -23 dBm) or too high, which indicates a fiber or attenuation problem rather than a faulty SFP. High temperature or out-of-range voltage suggests a hardware failure or environmental issue that must be addressed before the link can stabilize.

• ✓

  If optical power is outside the acceptable range, clean fiber connectors and re-test; if levels are normal but the link is still down, replace the SFP transceiver.

  Why this is correct

  Correct: This is the appropriate corrective action after DDM data shows optical power outside the acceptable range. Dirty fiber connectors are a leading cause of low RX power, so carefully cleaning both the patch cord ends and the SFP's optical ports with a lint-free swab and re-testing often resolves the issue. If the power levels are within specification but the link remains down, the SFP itself is likely defective and should be replaced with a compatible transceiver, after also ruling out a faulty cable or peer port.