The correct fix is to use a mode-conditioning patch cable between the SFP and the fiber patch panel. This resolves CRC errors on fiber links caused by a mode mismatch when a 1000BASE-LX SFP, designed for single-mode fiber, is connected to multimode fiber. The LX transceiver’s narrow 1310 nm laser beam, when launched into the larger core of multimode fiber, excites multiple propagation modes, creating differential mode delay that corrupts the signal and drives up CRC errors. A mode-conditioning patch cable offsets the laser launch off-center, suppressing higher-order modes and restoring signal integrity. On the CCNA 200-301 v2 exam, this scenario tests your understanding of fiber-optic transceiver compatibility and the specific Cisco best practice for LX SFPs on multimode links. A common trap is to replace the SFP with a 1000BASE-SX model, but SX uses 850 nm and is incompatible with the longer LX wavelength. Memory tip: think “LX on MMF needs a mode-conditioning patch to fix the mismatch.”
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 gigabitethernet 0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
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 off, input flow-control is off
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 00:05:00
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
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
A network engineer is troubleshooting a link between two Cisco switches that are connected via a 1000BASE-LX SFP transceiver. Hosts on VLAN 10 on Switch A cannot ping the default gateway on Switch B. The interface on Switch A is up, but the engineer notices CRC errors incrementing. What should the engineer do to resolve this issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a mode-conditioning patch cable between the SFP and the fiber patch panel
The 1000BASE-LX SFP transceiver operates at 1310 nm over single-mode fiber (SMF). When connected to multimode fiber (MMF), the larger core diameter causes excessive modal dispersion, leading to CRC errors. A mode-conditioning patch cable (mode-conditioning patch cord) launches the laser off-center into the MMF core, reducing differential mode delay and restoring proper signal integrity. This is a known Cisco best practice for LX transceivers on MMF links.
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.
✗
Replace the SFP with a 1000BASE-SX transceiver
Why it's wrong here
1000BASE-SX is for multimode fiber only, but the transceiver is already LX (single-mode).
✓
Use a mode-conditioning patch cable between the SFP and the fiber patch panel
Why this is correct
A mode-conditioning patch cable corrects the mismatch when using a single-mode transceiver over multimode fiber, preventing signal dispersion.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure the same VLAN on both switch interfaces
Why it's wrong here
VLAN mismatch would cause the line protocol to be down, not CRC errors.
✗
Replace the fiber cable with a single-mode fiber cable
Why it's wrong here
While this would also fix the mismatch, the question asks for the most likely cause given the symptom of CRC errors.
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.
✓Use a mode-conditioning patch cable between the SFP and the fiber patch panelCorrect answer▾
Why this is correct
A mode-conditioning patch cable corrects the mismatch when using a single-mode transceiver over multimode fiber, preventing signal dispersion.
✗Replace the SFP with a 1000BASE-SX transceiverWrong answer — click to see why▾
Why this is wrong here
Using SX would not fix the issue because the fiber is multimode and LX is designed for longer distances; the problem is the cable type mismatch, not the transceiver type.
✗Configure the same VLAN on both switch interfacesWrong answer — click to see why▾
Why this is wrong here
The interface is up/up, so VLAN mismatch is not the issue.
✗Replace the fiber cable with a single-mode fiber cableWrong answer — click to see why▾
Why this is wrong here
Although replacing with single-mode fiber would work, the exhibit does not show CRC errors, so the problem is more likely a mode-conditioning issue.
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 misconception that CRC errors always indicate a bad cable or transceiver, when in fact the root cause can be a mode mismatch between LX optics and multimode fiber that is fixed with a mode-conditioning patch cable rather than replacing the fiber or transceiver.
Detailed technical explanation
How to think about this question
1000BASE-LX transceivers use a 1310 nm laser with a narrow spectral width, optimized for single-mode fiber (9 µm core). When connected to multimode fiber (50 µm or 62.5 µm core), the laser light couples into multiple modes, causing differential mode delay (DMD) that corrupts the signal. A mode-conditioning patch cable offsets the fiber core alignment by approximately 10–20 µm, ensuring the laser excites only a subset of modes, which eliminates DMD and allows error-free transmission over distances up to 550 m on MMF. Cisco documentation (e.g., Cabling Guidelines for 1000BASE-LX/LH) explicitly recommends this solution.
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: Use a mode-conditioning patch cable between the SFP and the fiber patch panel — The 1000BASE-LX SFP transceiver operates at 1310 nm over single-mode fiber (SMF). When connected to multimode fiber (MMF), the larger core diameter causes excessive modal dispersion, leading to CRC errors. A mode-conditioning patch cable (mode-conditioning patch cord) launches the laser off-center into the MMF core, reducing differential mode delay and restoring proper signal integrity. This is a known Cisco best practice for LX transceivers on MMF links.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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