Question 1hardmultiple choice
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Question 27 of 1,527
hardmultiple choiceObjective-mapped
CCNA Practice Question: A network technician is troubleshooting a newly…
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 gigabitethernet1/0/1
GigabitEthernet1/0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
Description: Fiber uplink to SwitchB
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, 1000Mbps, link type is auto, media type is 10/100/1000BaseTX
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: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 1000 bits/sec, 2 packets/sec
5 minute output rate 1000 bits/sec, 2 packets/sec
1250 packets input, 125000 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
1250 input errors, 1250 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
1250 packets output, 125000 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
SwitchA# show interfaces transceiver details gigabitethernet1/0/1
Transceiver Type: SFP
Product Name: SFP-GE-SX-MM850
Serial Number: FNS1234567
Connector type: LC
Wavelength: 850nm
Transfer Distance: 550m (50/125 µm OM3)
Diagnostic Monitoring: Yes
Temperature: 45.2 Celsius
Voltage: 3.30 Volts
Current: 8.5 mA
Output Power: -3.5 dBm
Receive Power: -20.1 dBm
Transmit Power High Threshold: -1.0 dBm
Transmit Power Low Threshold: -9.5 dBm
Receive Power High Threshold: 0.0 dBm
Receive Power Low Threshold: -17.0 dBmA network technician is troubleshooting a newly installed fiber link between two Cisco Catalyst 9300 switches. The link is up, but the interface shows excessive CRC errors and input errors. The technician runs 'show interfaces' and 'show interfaces transceiver details' on the suspect interface. Based on the output, what is the most likely cause of the errors?
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 receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.
The receive power of -20.1 dBm is below the low threshold of -17.0 dBm for the SX SFP, indicating a weak signal. This causes high CRC errors due to bit errors. The media type shown as 10/100/1000BaseTX suggests the interface is incorrectly configured for copper instead of fiber, but the SFP transceiver is present and the link is up, so the primary issue is the low receive power. The distance of 550m is within limits for OM3 fiber at 1 Gbps, but the signal is too weak, likely due to a dirty connector or attenuation.
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 for full duplex but the switchport is set to auto-negotiation, causing a duplex mismatch.
Why it's wrong here
The output shows Full Duplex and 1000Mbps, with link type auto. There is no evidence of duplex mismatch, as no collisions or late collisions are present.
- ✗
The SFP transceiver is faulty because the transmit power is too low at -3.5 dBm.
Why it's wrong here
The transmit power of -3.5 dBm is within the normal range (-1.0 to -9.5 dBm), so the SFP is transmitting correctly.
- ✗
The fiber cable is too long for the SX SFP, exceeding the distance limit.
Why it's wrong here
The SX SFP supports up to 550m on OM3 fiber, and the link is operational, so distance is not the issue.
- ✓
The receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.
Why this is correct
The receive power of -20.1 dBm is below the low threshold of -17.0 dBm, indicating a weak signal that causes CRC errors. This is often due to dirty connectors, damaged fiber, or excessive attenuation.
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.
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 receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.Correct answer▾
Why this is correct
The receive power of -20.1 dBm is below the low threshold of -17.0 dBm, indicating a weak signal that causes CRC errors. This is often due to dirty connectors, damaged fiber, or excessive attenuation.
✗The interface is configured for full duplex but the switchport is set to auto-negotiation, causing a duplex mismatch.Wrong answer — click to see why▾
Why this is wrong here
This is incorrect because the interface is operating at full duplex with no collisions, and the CRC errors are not typically caused by duplex mismatch.
✗The SFP transceiver is faulty because the transmit power is too low at -3.5 dBm.Wrong answer — click to see why▾
Why this is wrong here
This is incorrect because the transmit power is within thresholds; the issue is on the receive side.
✗The fiber cable is too long for the SX SFP, exceeding the distance limit.Wrong answer — click to see why▾
Why this is wrong here
This is incorrect because the distance limit is not exceeded; the problem is low receive power.
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 output shows Full Duplex and 1000Mbps, with link type auto. There is no evidence of duplex mismatch, as no collisions or late collisions are present.
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 receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation. — The receive power of -20.1 dBm is below the low threshold of -17.0 dBm for the SX SFP, indicating a weak signal. This causes high CRC errors due to bit errors. The media type shown as 10/100/1000BaseTX suggests the interface is incorrectly configured for copper instead of fiber, but the SFP transceiver is present and the link is up, so the primary issue is the low receive power. The distance of 550m is within limits for OM3 fiber at 1 Gbps, but the signal is too weak, likely due to a dirty connector or attenuation.
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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