The answer is to remove manual speed and duplex settings, enable auto-negotiation, and replace the SFP with a long-reach module like 1000BASE-ZX. This resolves packet loss caused by a speed duplex mismatch, which occurs when one interface is manually set while the peer uses auto-negotiation, leading to a duplex mismatch that corrupts frames. Additionally, the standard SFP cannot support the required 40 km over single-mode fiber; only a 1000BASE-ZX transceiver is rated for that distance, whereas 1000BASE-LX/LH maxes out at 10 km. On the CCNA 200-301 v2 exam, this tests your ability to troubleshoot physical-layer issues and select the correct transceiver for a given distance—a common trap is assuming any SFP works for any fiber run. Remember the mnemonic: “No speed, no duplex, no packet loss; for 40 km, think ZX.”
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.
Network Topology
You are troubleshooting a link between R1 (G0/0) and R2 (G0/0). The link is up but experiencing packet loss. You suspect an interface speed/duplex mismatch or SFP issue. Configure R1's interface to match the correct speed and duplex, and replace the SFP module if necessary to support a required distance of 40 km over single-mode fiber.
R1# show interfaces gigabitethernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Hardware is GigabitEthernet, address is aabb.cc00.0100 (bia aabb.cc00.0100)
Internet address is 192.0.2.1/30
MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 10Mb/s, 100BaseTX/FX
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:05, 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, 1 packets/sec
5 minute output rate 1000 bits/sec, 1 packets/sec
1020 packets input, 100200 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
1010 packets output, 99000 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
R1# show interfaces gigabitethernet 0/0 transceiver
Diagnostic Monitoring Information:
Temperature: 45 C
Voltage: 3.3 V
Current: 10 mA
Transmit Power: -3.1 dBm
Receive Power: -12.5 dBm
High Alarm: RX power low
Module Type: SFP
Connector: LC
Wavelength: 1310 nm
Distance: 10 km
Media: SMF
R1# show running-config interface gigabitethernet 0/0
Building configuration...
Current configuration : 120 bytes
!
interface GigabitEthernet0/0
ip address 192.0.2.1 255.255.255.252
duplex half
speed 10
end
A
Remove manual speed/duplex settings, enable auto-negotiation, and replace the SFP with a long-reach module (e.g., 1000BASE-ZX) that supports 40 km over single-mode fiber.
This action correctly addresses both the potential speed/duplex mismatch by reverting to auto-negotiation (the default on Cisco interfaces) and the distance requirement by replacing the SFP with one capable of 40 km on SMF.
B
Change the duplex to full and speed to 1000 Mbps manually, and keep the existing SFP.
Why wrong: This is incorrect because manually setting speed and duplex disables auto-negotiation, which can still cause mismatch if the remote end is set differently. Moreover, the existing SFP is rated for only 10 km, not the required 40 km, so it must be replaced.
C
Keep the manual speed/duplex settings but replace the SFP with a 1000BASE-SX module.
Why wrong: This is incorrect because keeping manual settings does not resolve the mismatch; the link partner likely uses auto-negotiation or different settings. Also, 1000BASE-SX is designed for multimode fiber (up to 550 m), not single-mode fiber, and cannot support 40 km.
D
Enable auto-negotiation on both ends but keep the existing SFP.
Why wrong: This is incorrect because while enabling auto-negotiation fixes the speed/duplex mismatch, the existing SFP is rated for only 10 km, which does not meet the 40 km requirement. The SFP must be replaced.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Remove manual speed/duplex settings, enable auto-negotiation, and replace the SFP with a long-reach module (e.g., 1000BASE-ZX) that supports 40 km over single-mode fiber.
The link is experiencing packet loss, likely due to a speed/duplex mismatch between the manually configured interface and the peer (which may use auto-negotiation) and/or an SFP module that cannot support the required 40 km distance over single-mode fiber. To resolve both potential issues, remove any manual speed and duplex settings to restore default auto-negotiation (using 'no speed' and 'no duplex'), and replace the SFP with a module rated for 40 km or more on SMF, such as a 1000BASE-ZX (or 1000BASE-EX) transceiver. Standard 1000BASE-LX/LH only reaches up to 10 km on SMF, so it is insufficient for this scenario.
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.
✓
Remove manual speed/duplex settings, enable auto-negotiation, and replace the SFP with a long-reach module (e.g., 1000BASE-ZX) that supports 40 km over single-mode fiber.
Why this is correct
This action correctly addresses both the potential speed/duplex mismatch by reverting to auto-negotiation (the default on Cisco interfaces) and the distance requirement by replacing the SFP with one capable of 40 km on SMF.
Related concept
Read the scenario before looking for a memorised answer.
✗
Change the duplex to full and speed to 1000 Mbps manually, and keep the existing SFP.
Why it's wrong here
This is incorrect because manually setting speed and duplex disables auto-negotiation, which can still cause mismatch if the remote end is set differently. Moreover, the existing SFP is rated for only 10 km, not the required 40 km, so it must be replaced.
✗
Keep the manual speed/duplex settings but replace the SFP with a 1000BASE-SX module.
Why it's wrong here
This is incorrect because keeping manual settings does not resolve the mismatch; the link partner likely uses auto-negotiation or different settings. Also, 1000BASE-SX is designed for multimode fiber (up to 550 m), not single-mode fiber, and cannot support 40 km.
✗
Enable auto-negotiation on both ends but keep the existing SFP.
Why it's wrong here
This is incorrect because while enabling auto-negotiation fixes the speed/duplex mismatch, the existing SFP is rated for only 10 km, which does not meet the 40 km requirement. The SFP must be replaced.
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.
✓Remove manual speed/duplex settings, enable auto-negotiation, and replace the SFP with a long-reach module (e.g., 1000BASE-ZX) that supports 40 km over single-mode fiber.Correct answer▾
Why this is correct
This action correctly addresses both the potential speed/duplex mismatch by reverting to auto-negotiation (the default on Cisco interfaces) and the distance requirement by replacing the SFP with one capable of 40 km on SMF.
✗Change the duplex to full and speed to 1000 Mbps manually, and keep the existing SFP.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that manually setting speed/duplex does not guarantee compatibility with the remote end and does not address the SFP distance limitation.
Why candidates choose this
Candidates might think that manually forcing 1000 Mbps full-duplex will fix the mismatch, but they overlook the SFP requirement and the risk of mismatch if the remote end uses auto-negotiation.
✗Keep the manual speed/duplex settings but replace the SFP with a 1000BASE-SX module.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that 1000BASE-SX is for multimode fiber and short distances, not single-mode fiber at 40 km.
Why candidates choose this
Candidates might confuse SFP types and think any Gigabit SFP will work, but they ignore the fiber type and distance limitations.
✗Enable auto-negotiation on both ends but keep the existing SFP.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the SFP distance limitation is not addressed; auto-negotiation alone does not change the physical layer capability.
Why candidates choose this
Candidates might focus only on the mismatch issue and forget the distance requirement, assuming the SFP is adequate.
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.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
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.
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: Remove manual speed/duplex settings, enable auto-negotiation, and replace the SFP with a long-reach module (e.g., 1000BASE-ZX) that supports 40 km over single-mode fiber. — The link is experiencing packet loss, likely due to a speed/duplex mismatch between the manually configured interface and the peer (which may use auto-negotiation) and/or an SFP module that cannot support the required 40 km distance over single-mode fiber. To resolve both potential issues, remove any manual speed and duplex settings to restore default auto-negotiation (using 'no speed' and 'no duplex'), and replace the SFP with a module rated for 40 km or more on SMF, such as a 1000BASE-ZX (or 1000BASE-EX) transceiver. Standard 1000BASE-LX/LH only reaches up to 10 km on SMF, so it is insufficient for this scenario.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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