Question 1hardmultiple choice
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Question 1,160 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: Is troubleshooting a link between two Cisco…
This 200-301 practice question tests your understanding of 200-301 exam topics. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
SwitchA# show interfaces gigabitethernet1/0/1
GigabitEthernet1/0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
Description: Link to SwitchB
Internet address is 192.168.1.1/30
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, media type is SFP
output flow-control is unsupported, input flow-control is unsupported
Auto-negotiation is turned on
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out
SwitchA# show interfaces gigabitethernet1/0/1 transceiver
Diagnostic Monitoring Information (DOM) for SFP+ 10GBASE-LR
Temperature: 45 degrees C
Voltage: 3.3 V
Current: 15 mA
Output Power: 0.5 mW (-3 dBm)
Input Power: 0.01 mW (-20 dBm)
Laser bias current: 10 mA
SwitchB# show interfaces gigabitethernet1/0/1
GigabitEthernet1/0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is dddd.eeee.ffff (bia dddd.eeee.ffff)
Description: Link to SwitchA
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, media type is SFP
output flow-control is unsupported, input flow-control is unsupported
Auto-negotiation is turned on
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out
SwitchB# show interfaces gigabitethernet1/0/1 transceiver
Diagnostic Monitoring Information (DOM) for SFP+ 10GBASE-LR
Temperature: 42 degrees C
Voltage: 3.3 V
Current: 12 mA
Output Power: 0.4 mW (-4 dBm)
Input Power: 0.02 mW (-17 dBm)
Laser bias current: 8 mAA network engineer is troubleshooting a link between two Cisco Catalyst 9300 switches that are connected via a single-mode fiber optic cable. The link is up, but the interface counters show a high number of CRC errors and frame check sequence (FCS) errors. The interface is configured for 1000 Mbps and full duplex on both ends. What is the most likely cause of these errors?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Faulty or dirty fiber optic cable or connectors
The CRC and FCS errors on a fiber link with proper speed/duplex settings typically indicate physical layer issues such as faulty cabling, dirty connectors, or transceiver problems. The DOM output shows that the received power on SwitchA is -20 dBm, which is below the typical receive sensitivity for a 10GBASE-LR SFP+ module (usually around -14.4 dBm). This low received power can cause bit errors, even though the link is up. The likely cause is a damaged fiber cable or a failing transmitter on SwitchB, leading to excessive signal loss.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A speed or duplex mismatch between the two switches
Why it's wrong here
Both interfaces show full-duplex, 1000Mb/s, and auto-negotiation is on, so a mismatch is not present.
- ✓
Faulty or dirty fiber optic cable or connectors
Why this is correct
The low received power (-20 dBm on SwitchA) indicates excessive loss, likely due to a damaged cable or dirty connectors.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Incorrect SFP+ module type inserted in the switch
Why it's wrong here
Both DOM outputs show SFP+ 10GBASE-LR modules, which are compatible with single-mode fiber and 1000 Mbps operation.
- ✗
Excessive cable length beyond the distance limit
Why it's wrong here
While excessive length could cause low power, the DOM output does not indicate that the cable is too long; the low power could be due to other factors like damage.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Faulty or dirty fiber optic cable or connectorsCorrect answer▾
Why this is correct
The low received power (-20 dBm on SwitchA) indicates excessive loss, likely due to a damaged cable or dirty connectors.
✗A speed or duplex mismatch between the two switchesWrong answer — click to see why▾
Why this is wrong here
The exhibit confirms consistent speed and duplex settings.
✗Incorrect SFP+ module type inserted in the switchWrong answer — click to see why▾
Why this is wrong here
The modules are correctly identified and match the fiber type.
✗Excessive cable length beyond the distance limitWrong answer — click to see why▾
Why this is wrong here
There is no evidence of the cable length in the exhibit, and the low power is more indicative of a physical fault rather than simply exceeding distance limits.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Command / output trap
Both interfaces show full-duplex, 1000Mb/s, and auto-negotiation is on, so a mismatch is not present.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
- Use explanations to understand the rule behind the answer.
TExam Day Tips
- Underline the problem statement mentally.
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 200-301 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which 200-301 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Read the scenario before looking for a memorised answer.
What is the correct answer to this question?
The correct answer is: Faulty or dirty fiber optic cable or connectors — The CRC and FCS errors on a fiber link with proper speed/duplex settings typically indicate physical layer issues such as faulty cabling, dirty connectors, or transceiver problems. The DOM output shows that the received power on SwitchA is -20 dBm, which is below the typical receive sensitivity for a 10GBASE-LR SFP+ module (usually around -14.4 dBm). This low received power can cause bit errors, even though the link is up. The likely cause is a damaged fiber cable or a failing transmitter on SwitchB, leading to excessive signal loss.
What should I do if I get this 200-301 question wrong?
Identify which 200-301 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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