The answer is a faulty or incompatible SFP module. This is correct because the `show controllers` command on the server farm switch reveals physical-layer input errors like CRC, framing, or alignment errors, which are classic symptoms of signal degradation or electrical issues introduced by a defective SFP, even when the link state appears up. Since the user switch’s uplink shows no errors, the problem is isolated to the server farm switch’s interface, ruling out cable or distance issues. On the CCNA 200-301 v2 exam, this scenario tests your ability to differentiate between Layer 1 problems visible via `show controllers` versus Layer 2 errors seen with `show interfaces`. A common trap is assuming a link that is up/up is error-free; remember that SFPs can fail partially, causing input errors without a full outage. Memory tip: “Controllers for the core, interfaces for the frame”—use `show controllers` to catch physical-layer SFP faults that `show interfaces` might miss.
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
ServerFarm-SW1# show controllers GigabitEthernet0/1
Interface GigabitEthernet0/1
Hardware is Gigabit Ethernet
Media Type: 1000BaseSX SFP
Link Status: Up
Auto-negotiation: off
Speed: 1000 Mbps
Duplex: Full
Transmit Errors: 0
Receive Errors: 0
CRC Errors: 0
Frame Errors: 0
Runts: 0
Giants: 0
Input Errors: 0
Output Errors: 0
Collisions: 0
Late Collisions: 0
Deferred: 0
Lost Carrier: 0
No Carrier: 0
FCS Errors: 0
Symbol Errors: 0
ServerFarm-SW1# show interfaces GigabitEthernet0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is 0011.2233.4455 (bia 0011.2233.4455)
Description: Uplink to Access-SW1
Internet address is 192.168.1.2/30
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 SX
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: 0/2000/0/0 (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
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 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, 2 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
ServerFarm-SW1# show interfaces status
Port Name Status Vlan Duplex Speed Type
Gi0/1 Uplink to Access-SW1 connected trunk full 1000 1000BaseSX SFP
Gi0/2 Server1 connected 100 full 1000 10/100/1000BaseTX
Gi0/3 Server2 connected 100 full 1000 10/100/1000BaseTX
A network engineer notices that users on VLAN 100 are experiencing intermittent connectivity to the server farm. The switch connecting these users shows no errors on the uplink interface, but the server farm switch reports a high number of input errors on its connected interface. The engineer runs 'show controllers' on the server farm switch. What is the most likely cause of the issue?
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.
ServerFarm-SW1# show controllers GigabitEthernet0/1
Interface GigabitEthernet0/1
Hardware is Gigabit Ethernet
Media Type: 1000BaseSX SFP
Link Status: Up
Auto-negotiation: off
Speed: 1000 Mbps
Duplex: Full
Transmit Errors: 0
Receive Errors: 0
CRC Errors: 0
Frame Errors: 0
Runts: 0
Giants: 0
Input Errors: 0
Output Errors: 0
Collisions: 0
Late Collisions: 0
Deferred: 0
Lost Carrier: 0
No Carrier: 0
FCS Errors: 0
Symbol Errors: 0
ServerFarm-SW1# show interfaces GigabitEthernet0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is 0011.2233.4455 (bia 0011.2233.4455)
Description: Uplink to Access-SW1
Internet address is 192.168.1.2/30
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 SX
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: 0/2000/0/0 (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
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 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, 2 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
ServerFarm-SW1# show interfaces status
Port Name Status Vlan Duplex Speed Type
Gi0/1 Uplink to Access-SW1 connected trunk full 1000 1000BaseSX SFP
Gi0/2 Server1 connected 100 full 1000 10/100/1000BaseTX
Gi0/3 Server2 connected 100 full 1000 10/100/1000BaseTX
A
The interface is configured with the wrong duplex setting.
Why wrong: The show output confirms Full-duplex on both ends, so duplex mismatch is not the issue.
B
The SFP module is faulty or incompatible with the cable type.
The 'show controllers' output shows the media type as 1000BaseSX SFP with auto-negotiation off, but the interface is reporting no errors. However, the other switch sees input errors. This points to a hardware issue with the SFP, such as a faulty module or a mismatch between the SFP and the fiber cable (e.g., using a single-mode SFP with multi-mode fiber).
C
The cable is too long, causing attenuation.
Why wrong: While cable length can cause errors, the 'show controllers' output does not indicate any specific error counters that would suggest attenuation, such as symbol errors or FCS errors.
D
Auto-negotiation is disabled, causing a speed mismatch.
Why wrong: Speed is set to 1000 Mbps on both ends, and auto-negotiation is off, which is normal for fiber connections. Speed mismatch would cause the link to not come up or show errors, but the link is up and no errors are reported here.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The SFP module is faulty or incompatible with the cable type.
The 'show controllers' command on the server farm switch reveals physical-layer issues such as framing errors, CRC errors, or alignment errors, which are often caused by faulty or incompatible SFP modules. Since the uplink interface on the user switch shows no errors, the problem is isolated to the server farm switch's interface, and a faulty SFP can introduce signal degradation or electrical issues without necessarily causing complete link failure. Option B is correct because SFP incompatibility or defects commonly produce input errors at the physical layer, even when the link appears up.
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 with the wrong duplex setting.
Why it's wrong here
The show output confirms Full-duplex on both ends, so duplex mismatch is not the issue.
✓
The SFP module is faulty or incompatible with the cable type.
Why this is correct
The 'show controllers' output shows the media type as 1000BaseSX SFP with auto-negotiation off, but the interface is reporting no errors. However, the other switch sees input errors. This points to a hardware issue with the SFP, such as a faulty module or a mismatch between the SFP and the fiber cable (e.g., using a single-mode SFP with multi-mode fiber).
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.
✗
The cable is too long, causing attenuation.
Why it's wrong here
While cable length can cause errors, the 'show controllers' output does not indicate any specific error counters that would suggest attenuation, such as symbol errors or FCS errors.
✗
Auto-negotiation is disabled, causing a speed mismatch.
Why it's wrong here
Speed is set to 1000 Mbps on both ends, and auto-negotiation is off, which is normal for fiber connections. Speed mismatch would cause the link to not come up or show errors, but the link is up and no errors are reported here.
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 SFP module is faulty or incompatible with the cable type.Correct answer▾
Why this is correct
The 'show controllers' output shows the media type as 1000BaseSX SFP with auto-negotiation off, but the interface is reporting no errors. However, the other switch sees input errors. This points to a hardware issue with the SFP, such as a faulty module or a mismatch between the SFP and the fiber cable (e.g., using a single-mode SFP with multi-mode fiber).
✗The interface is configured with the wrong duplex setting.Wrong answer — click to see why▾
Why this is wrong here
The 'show controllers' output confirms Full-duplex on both ends, so a duplex mismatch is not the cause. Duplex mismatch would typically cause collisions or CRC errors, which are not indicated here.
Why candidates choose this
Students often associate input errors with duplex mismatch, as it is a common cause of errors on Ethernet links. However, the output clearly shows both ends are set to Full-duplex, ruling this out.
✗The cable is too long, causing attenuation.Wrong answer — click to see why▾
Why this is wrong here
While excessive cable length can cause attenuation and errors, the 'show controllers' output does not show specific error counters like symbol errors or FCS errors that would indicate attenuation. The link is up and no errors are reported on this switch, making cable length an unlikely cause.
Why candidates choose this
Cable length is a known cause of signal degradation and errors, so students might jump to this conclusion. However, the absence of related error counters and the fact that errors are only seen on one side suggest a hardware issue rather than a cable length problem.
✗Auto-negotiation is disabled, causing a speed mismatch.Wrong answer — click to see why▾
Why this is wrong here
Speed is set to 1000 Mbps on both ends, and auto-negotiation is off, which is normal for fiber connections. A speed mismatch would prevent the link from coming up or cause constant errors, but the link is up and no errors are reported on this switch.
Why candidates choose this
Auto-negotiation is often misunderstood; students may think disabling it always causes issues. However, for fiber Gigabit Ethernet, auto-negotiation is optional and speed is typically fixed. The absence of errors on this switch indicates speed mismatch is not the problem.
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 'show interfaces' (which shows input errors but not the specific physical-layer cause) and 'show controllers' (which reveals the exact physical-layer errors), leading candidates to mistakenly choose duplex mismatch or cable length issues without recognizing that the command output points to SFP or transceiver problems.
Trap categories for this question
Command / output trap
The show output confirms Full-duplex on both ends, so duplex mismatch is not the issue.
Detailed technical explanation
How to think about this question
The 'show controllers' command provides detailed physical-layer statistics, including CRC errors, framing errors, and symbol errors, which are often caused by faulty optics, dirty fiber connectors, or incompatible SFP modules. In real-world scenarios, using a third-party SFP in a Cisco switch can cause the interface to report high input errors due to electrical signaling differences, even if the link is up and auto-negotiation succeeds. Cisco switches also use DOM (Digital Optical Monitoring) to detect SFP issues, but a faulty SFP may still pass basic link detection while corrupting data at the physical layer.
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 SFP module is faulty or incompatible with the cable type. — The 'show controllers' command on the server farm switch reveals physical-layer issues such as framing errors, CRC errors, or alignment errors, which are often caused by faulty or incompatible SFP modules. Since the uplink interface on the user switch shows no errors, the problem is isolated to the server farm switch's interface, and a faulty SFP can introduce signal degradation or electrical issues without necessarily causing complete link failure. Option B is correct because SFP incompatibility or defects commonly produce input errors at the physical layer, even when the link appears up.
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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