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Question 570 of 1,052
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CCNA Practice Question: Notices that users on VLAN 100 are experiencing…
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
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 10/100/1000BaseTXA 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.
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
✓
The SFP module is faulty or incompatible with the cable type.
The 'show controllers' output indicates that the SFP media type is 1000BaseSX, and auto-negotiation is disabled. However, the 'show interfaces' and 'show interfaces status' both report the link as up with full duplex and 1000 Mbps. The problem is that the SFP is a fiber optic module (1000BaseSX), but the 'show interfaces status' shows the Type as 1000BaseSX SFP, which is correct for fiber. But the real issue is that the media type is SX (fiber), but the interface description and the fact that it's connected to a switch suggest it should be a copper link, or the SFP is mismatched. However, the key clue is that 'show controllers' shows 'Media Type: 1000BaseSX SFP' and 'Auto-negotiation: off'. For fiber optic links, auto-negotiation is typically not required, but if the other end expects it, or if the cable is actually copper (e.g., a mis-cabled SFP), this can cause intermittent issues. The most likely cause is a mismatch between the SFP module type and the actual cable (fiber vs. copper) or a faulty SFP. But given the symptoms of high input errors on the other switch and no errors here, the issue is likely a hardware problem with the SFP module itself.
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.
- ✗
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
Access ports place end devices into a single VLAN.
- ✗
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
Duplex is correctly set to full on both sides.
✗The cable is too long, causing attenuation.Wrong answer — click to see why▾
Why this is wrong here
No errors are reported on this interface, so cable length is unlikely the primary cause.
✗Auto-negotiation is disabled, causing a speed mismatch.Wrong answer — click to see why▾
Why this is wrong here
Speed is consistent at 1000 Mbps, and auto-negotiation is not required for fiber.
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.
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
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: The SFP module is faulty or incompatible with the cable type. — The 'show controllers' output indicates that the SFP media type is 1000BaseSX, and auto-negotiation is disabled. However, the 'show interfaces' and 'show interfaces status' both report the link as up with full duplex and 1000 Mbps. The problem is that the SFP is a fiber optic module (1000BaseSX), but the 'show interfaces status' shows the Type as 1000BaseSX SFP, which is correct for fiber. But the real issue is that the media type is SX (fiber), but the interface description and the fact that it's connected to a switch suggest it should be a copper link, or the SFP is mismatched. However, the key clue is that 'show controllers' shows 'Media Type: 1000BaseSX SFP' and 'Auto-negotiation: off'. For fiber optic links, auto-negotiation is typically not required, but if the other end expects it, or if the cable is actually copper (e.g., a mis-cabled SFP), this can cause intermittent issues. The most likely cause is a mismatch between the SFP module type and the actual cable (fiber vs. copper) or a faulty SFP. But given the symptoms of high input errors on the other switch and no errors here, the issue is likely a hardware problem with the SFP module itself.
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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