Question 1hardmultiple choice
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Question 1,211 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 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 outA 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 is the most likely cause of this 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.
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
✓
Use a mode-conditioning patch cable between the SFP and the fiber patch panel
The issue is a mismatch between the SFP transceiver type and the fiber cable. The show interface output shows no CRC errors, but the problem is that the link is up with no errors, yet hosts cannot ping. This is caused by using a single-mode SFP (1000BASE-LX) with a multimode fiber cable, which can cause intermittent connectivity or packet loss due to signal dispersion. The correct fix is to use a mode-conditioning patch cable to properly couple the single-mode transceiver to multimode fiber. Option B is correct because it addresses the physical layer mismatch. Option A is wrong because the interface is up and duplex is auto-negotiated correctly. Option C is wrong because VLAN mismatch would cause the interface to be up but protocol down. Option D is wrong because CRC errors are not present in the output.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
- ✗
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: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
What to study next
Got this wrong? Here's your next step.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Access ports place end devices into a single VLAN.
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 issue is a mismatch between the SFP transceiver type and the fiber cable. The show interface output shows no CRC errors, but the problem is that the link is up with no errors, yet hosts cannot ping. This is caused by using a single-mode SFP (1000BASE-LX) with a multimode fiber cable, which can cause intermittent connectivity or packet loss due to signal dispersion. The correct fix is to use a mode-conditioning patch cable to properly couple the single-mode transceiver to multimode fiber. Option B is correct because it addresses the physical layer mismatch. Option A is wrong because the interface is up and duplex is auto-negotiated correctly. Option C is wrong because VLAN mismatch would cause the interface to be up but protocol down. Option D is wrong because CRC errors are not present in the output.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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