The answer is low receive power, most likely due to a dirty or damaged fiber connector or excessive attenuation. When a fiber link is up but accumulating excessive CRC errors and input errors, the root cause is often insufficient optical signal reaching the receiver. The 'show interfaces transceiver details' command reveals the receive power level; if it falls below the receiver’s sensitivity threshold, the interface cannot reliably decode bits, leading to cyclic redundancy check failures even though the physical link remains active. On the CCNA 200-301 v2 exam, this scenario tests your ability to differentiate between physical-layer issues and higher-layer problems—a common trap is assuming the link being up means the signal is clean. Remember that CRC errors on fiber almost always point to optical power problems, not duplex mismatches or cable length. Memory tip: “CRC on glass? Check the power—dirt or loss will make it sour.”
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. 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: Fiber uplink 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, 1000Mbps, link type is auto, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 1000 bits/sec, 2 packets/sec
5 minute output rate 1000 bits/sec, 2 packets/sec
1250 packets input, 125000 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
1250 input errors, 1250 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
1250 packets output, 125000 bytes, 0 underruns
0 output errors, 0 collisions, 1 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 transceiver details gigabitethernet1/0/1
Transceiver Type: SFP
Product Name: SFP-GE-SX-MM850
Serial Number: FNS1234567
Connector type: LC
Wavelength: 850nm
Transfer Distance: 550m (50/125 µm OM3)
Diagnostic Monitoring: Yes
Temperature: 45.2 Celsius
Voltage: 3.30 Volts
Current: 8.5 mA
Output Power: -3.5 dBm
Receive Power: -20.1 dBm
Transmit Power High Threshold: -1.0 dBm
Transmit Power Low Threshold: -9.5 dBm
Receive Power High Threshold: 0.0 dBm
Receive Power Low Threshold: -17.0 dBm
A network technician is troubleshooting a newly installed fiber link between two Cisco Catalyst 9300 switches. The link is up, but the interface shows excessive CRC errors and input errors. The technician runs 'show interfaces' and 'show interfaces transceiver details' on the suspect interface. Based on the output, what is the most likely cause of the 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.
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: Fiber uplink 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, 1000Mbps, link type is auto, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 1000 bits/sec, 2 packets/sec
5 minute output rate 1000 bits/sec, 2 packets/sec
1250 packets input, 125000 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
1250 input errors, 1250 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
1250 packets output, 125000 bytes, 0 underruns
0 output errors, 0 collisions, 1 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 transceiver details gigabitethernet1/0/1
Transceiver Type: SFP
Product Name: SFP-GE-SX-MM850
Serial Number: FNS1234567
Connector type: LC
Wavelength: 850nm
Transfer Distance: 550m (50/125 µm OM3)
Diagnostic Monitoring: Yes
Temperature: 45.2 Celsius
Voltage: 3.30 Volts
Current: 8.5 mA
Output Power: -3.5 dBm
Receive Power: -20.1 dBm
Transmit Power High Threshold: -1.0 dBm
Transmit Power Low Threshold: -9.5 dBm
Receive Power High Threshold: 0.0 dBm
Receive Power Low Threshold: -17.0 dBm
A
The interface is configured for full duplex but the switchport is set to auto-negotiation, causing a duplex mismatch.
Why wrong: The output shows Full Duplex and 1000Mbps, with link type auto. There is no evidence of duplex mismatch, as no collisions or late collisions are present.
B
The SFP transceiver is faulty because the transmit power is too low at -3.5 dBm.
Why wrong: The transmit power of -3.5 dBm is within the normal range (-1.0 to -9.5 dBm), so the SFP is transmitting correctly.
C
The fiber cable is too long for the SX SFP, exceeding the distance limit.
Why wrong: The SX SFP supports up to 550m on OM3 fiber, and the link is operational, so distance is not the issue.
D
The receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.
The receive power of -20.1 dBm is below the low threshold of -17.0 dBm, indicating a weak signal that causes CRC errors. This is often due to dirty connectors, damaged fiber, or excessive attenuation.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.
The output from 'show interfaces transceiver details' would show the receive power level. A receive power that is too low (e.g., below the receiver sensitivity threshold) indicates excessive signal loss, often due to dirty or damaged fiber connectors, poor splices, or excessive cable attenuation. This causes bit errors that manifest as CRC and input errors, even though the link is physically up. Option D correctly identifies this as the most likely cause.
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.
✗
The interface is configured for full duplex but the switchport is set to auto-negotiation, causing a duplex mismatch.
Why it's wrong here
The output shows Full Duplex and 1000Mbps, with link type auto. There is no evidence of duplex mismatch, as no collisions or late collisions are present.
✗
The SFP transceiver is faulty because the transmit power is too low at -3.5 dBm.
Why it's wrong here
The transmit power of -3.5 dBm is within the normal range (-1.0 to -9.5 dBm), so the SFP is transmitting correctly.
✗
The fiber cable is too long for the SX SFP, exceeding the distance limit.
Why it's wrong here
The SX SFP supports up to 550m on OM3 fiber, and the link is operational, so distance is not the issue.
✓
The receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.
Why this is correct
The receive power of -20.1 dBm is below the low threshold of -17.0 dBm, indicating a weak signal that causes CRC errors. This is often due to dirty connectors, damaged fiber, or excessive attenuation.
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.
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.
✓The receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.Correct answer▾
Why this is correct
The receive power of -20.1 dBm is below the low threshold of -17.0 dBm, indicating a weak signal that causes CRC errors. This is often due to dirty connectors, damaged fiber, or excessive attenuation.
✗The interface is configured for full duplex but the switchport is set to auto-negotiation, causing a duplex mismatch.Wrong answer — click to see why▾
Why this is wrong here
The interface shows Full Duplex and 1000 Mbps with no collisions or late collisions, which indicates no duplex mismatch. Duplex mismatch typically causes collisions and late collisions, which are absent here.
Why candidates choose this
Students often associate CRC errors with duplex mismatch, but CRC errors are more commonly caused by physical layer issues like signal degradation or faulty hardware, not duplex mismatch.
✗The SFP transceiver is faulty because the transmit power is too low at -3.5 dBm.Wrong answer — click to see why▾
Why this is wrong here
The transmit power of -3.5 dBm is within the normal range for SX SFP (-1.0 to -9.5 dBm), so the SFP is transmitting correctly. Low transmit power would not cause CRC errors if it is within specifications.
Why candidates choose this
Students may think that any low power reading indicates a faulty SFP, but the key is to compare against the threshold. Transmit power is often less critical than receive power for link errors.
✗The fiber cable is too long for the SX SFP, exceeding the distance limit.Wrong answer — click to see why▾
Why this is wrong here
The SX SFP supports up to 550m on OM3 fiber, and the link is operational, so distance is not the issue. Excessive CRC errors due to distance would typically cause the link to be down or unstable.
Why candidates choose this
Students may assume that fiber distance limits are always the cause of errors, but the link being up and the specific error pattern (CRC) points more to signal attenuation or connector issues.
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
Cisco often tests the distinction between transmit power and receive power, trapping candidates who assume a low transmit power is the root cause, when in fact the receive power is the critical metric for signal integrity at the far end.
Trap categories for this question
Command / output trap
The output shows Full Duplex and 1000Mbps, with link type auto. There is no evidence of duplex mismatch, as no collisions or late collisions are present.
Detailed technical explanation
How to think about this question
CRC errors on a fiber interface indicate that the received frame's frame check sequence (FCS) does not match the calculated value, meaning bits were corrupted during transmission. This is often caused by a weak or noisy optical signal at the receiver, where the optical power falls below the receiver's sensitivity (typically around -14 dBm to -20 dBm for SX SFPs). The 'show interfaces transceiver details' command provides the actual receive power in dBm, allowing comparison to the SFP's specified receiver sensitivity and saturation levels.
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.
TExam Day Tips
→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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation. — The output from 'show interfaces transceiver details' would show the receive power level. A receive power that is too low (e.g., below the receiver sensitivity threshold) indicates excessive signal loss, often due to dirty or damaged fiber connectors, poor splices, or excessive cable attenuation. This causes bit errors that manifest as CRC and input errors, even though the link is physically up. Option D correctly identifies this as the most likely cause.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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