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Question 343 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: A network technician is troubleshooting a link…
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
SW1# show interfaces gigabitethernet0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
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 1000BaseLX
output flow-control is off, input flow-control is off
...
SW2# show interfaces gigabitethernet0/1
GigabitEthernet0/1 is down, line protocol is down
Hardware is Gigabit Ethernet, address is dddd.eeee.ffff (bia dddd.eeee.ffff)
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 1000BaseSX
output flow-control is off, input flow-control is off
...A network technician is troubleshooting a link between two Cisco switches, SW1 and SW2, connected via a single-mode fiber optic cable. The interface on SW1 is up/up, but the interface on SW2 remains down/down. The technician has verified that the fiber cable is not damaged and that the SFP modules are correctly seated. Which configuration change should the technician make to resolve the issue?
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
✓
Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP.
The issue is a mismatch in the SFP transceiver types. SW1 is using a 1000BaseLX SFP (single-mode fiber) while SW2 is using a 1000BaseSX SFP (multimode fiber). Although single-mode fiber can be used with LX optics over longer distances, the SX SFP on SW2 is designed for multimode fiber and will not properly communicate with the LX SFP on SW1 over single-mode fiber. Replacing the SX SFP on SW2 with an LX SFP ensures both ends use the same physical layer standard, allowing the link to come up. The other options address issues that are not indicated by the output: speed/duplex mismatches would show a different symptom or be auto-negotiated, MDIX misconfiguration is irrelevant for fiber, and VLAN mismatch would not cause the interface to be down/down.
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.
- ✗
Configure the interface on SW2 to use the same speed and duplex settings as SW1.
Why it's wrong here
Both interfaces are already set to Full Duplex and 1000Mbps, and fiber links typically auto-negotiate correctly. This is not the root cause.
- ✓
Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP.
Why this is correct
The SFP types do not match—SW1 is LX (single-mode) and SW2 is SX (multimode). Using a consistent LX SFP on both ends ensures proper communication over single-mode fiber.
Related concept
Access ports place end devices into a single VLAN.
- ✗
Enable MDIX on both interfaces to allow automatic crossover detection.
Why it's wrong here
MDIX is used for copper Ethernet cables to correct for straight-through vs. crossover issues. Fiber optic connections do not use MDIX.
- ✗
Change the VLAN assignment on SW2's interface to match that of SW1.
Why it's wrong here
VLAN mismatch would cause the interface to be up/up but not forward traffic, not the down/down state seen here. The issue is at Layer 1.
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.
✓Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP.Correct answer▾
Why this is correct
The SFP types do not match—SW1 is LX (single-mode) and SW2 is SX (multimode). Using a consistent LX SFP on both ends ensures proper communication over single-mode fiber.
✗Configure the interface on SW2 to use the same speed and duplex settings as SW1.Wrong answer — click to see why▾
Why this is wrong here
The interface shows the same speed and duplex settings are already in place, so changing them would not resolve the down/down state.
✗Enable MDIX on both interfaces to allow automatic crossover detection.Wrong answer — click to see why▾
Why this is wrong here
This setting is irrelevant for fiber optic links and would not affect the interface state.
✗Change the VLAN assignment on SW2's interface to match that of SW1.Wrong answer — click to see why▾
Why this is wrong here
A VLAN mismatch would not prevent the interface from coming up; it would only affect Layer 2 or Layer 3 connectivity after the link is established.
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: Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP. — The issue is a mismatch in the SFP transceiver types. SW1 is using a 1000BaseLX SFP (single-mode fiber) while SW2 is using a 1000BaseSX SFP (multimode fiber). Although single-mode fiber can be used with LX optics over longer distances, the SX SFP on SW2 is designed for multimode fiber and will not properly communicate with the LX SFP on SW1 over single-mode fiber. Replacing the SX SFP on SW2 with an LX SFP ensures both ends use the same physical layer standard, allowing the link to come up. The other options address issues that are not indicated by the output: speed/duplex mismatches would show a different symptom or be auto-negotiated, MDIX misconfiguration is irrelevant for fiber, and VLAN mismatch would not cause the interface to be down/down.
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.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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