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Question 166 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: A network technician is troubleshooting…
This 200-301 practice question tests your understanding of a network technician is troubleshooting…. 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
Switch1# show interfaces gigabitethernet0/1
GigabitEthernet0/1 is up, line protocol is down (err-disabled)
Hardware is Gigabit Ethernet, address is aabb.cc00.0101 (bia aabb.cc00.0101)
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 1000BaseT
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:02, output 00:00:01, 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 15000 bits/sec, 18 packets/sec
5 minute output rate 14000 bits/sec, 17 packets/sec
12345 packets input, 987654 bytes, 0 no buffer
Received 123 broadcasts (0 multicast)
0 runts, 0 giants, 0 throttles
12345 input errors, 12345 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
12345 packets output, 987654 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
Switch1# show interfaces status
Port Name Status Vlan Duplex Speed Type
Gi0/1 err-disabled 1 full 1000 1000BaseT
Switch1# show controllers gigabitethernet0/1
Interface GigabitEthernet0/1
Hardware is Gigabit Ethernet
Address: aabb.cc00.0101 (bia aabb.cc00.0101)
...
Media Type: 1000BaseT, media selected: none
Auto-negotiation: enabled
Link partner capability: 1000BaseT FD
Link partner advertisement: 1000BaseT FD
...A network technician is troubleshooting intermittent connectivity on a trunk link between two Cisco switches. The output of 'show interfaces gigabitethernet0/1' reveals a high number of runts and giants, along with CRC errors. Additionally, 'show interfaces status' shows the interface is in err-disabled state. What is the most likely cause of these issues?
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
✓
There is a duplex mismatch between the switch and the connected device, with one side set to half-duplex.
The high number of CRC errors, runts, and giants in the show interfaces output, combined with the interface being in err-disabled state, indicates a duplex mismatch. The show controllers output reveals that auto-negotiation is enabled, but the switch is operating at full-duplex while the link partner advertises only full-duplex capability. However, the presence of runts and giants suggests that one side is misconfigured for half-duplex, causing collisions and frame errors. The most common cause is a misconfigured duplex setting on the remote device, leading to a duplex mismatch. This mismatch causes the interface to go err-disabled due to excessive errors.
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 cable is faulty or exceeds the maximum length for 1000BaseT.
Why it's wrong here
While faulty cables can cause errors, the high number of CRC errors and runts/giants specifically point to a duplex mismatch rather than a cable issue.
- ✓
There is a duplex mismatch between the switch and the connected device, with one side set to half-duplex.
Why this is correct
The combination of CRC errors, runts, giants, and err-disabled state is classic for a duplex mismatch. The show controllers output shows auto-negotiation enabled, but the presence of these errors indicates that the link partner is likely configured for half-duplex, causing frame collisions and errors.
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 interface is receiving excessive broadcast traffic, causing the input queue to overflow.
Why it's wrong here
Broadcast storms typically cause high CPU usage and input queue drops, not specifically CRC errors or runts/giants.
- ✗
The switch port is configured with the wrong VLAN, causing frame mismatches.
Why it's wrong here
Incorrect VLAN configuration would cause traffic to be dropped or misrouted, but not generate CRC errors or runts/giants.
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.
✓There is a duplex mismatch between the switch and the connected device, with one side set to half-duplex.Correct answer▾
Why this is correct
The combination of CRC errors, runts, giants, and err-disabled state is classic for a duplex mismatch. The show controllers output shows auto-negotiation enabled, but the presence of these errors indicates that the link partner is likely configured for half-duplex, causing frame collisions and errors.
✗The cable is faulty or exceeds the maximum length for 1000BaseT.Wrong answer — click to see why▾
Why this is wrong here
A cable fault would typically cause more physical layer errors like alignment or carrier errors, not specifically runts and giants.
✗The interface is receiving excessive broadcast traffic, causing the input queue to overflow.Wrong answer — click to see why▾
Why this is wrong here
The show interfaces output does not show input queue drops or high broadcast counts; the errors are at the data-link layer.
✗The switch port is configured with the wrong VLAN, causing frame mismatches.Wrong answer — click to see why▾
Why this is wrong here
VLAN mismatches typically result in connectivity issues without physical layer errors.
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: There is a duplex mismatch between the switch and the connected device, with one side set to half-duplex. — The high number of CRC errors, runts, and giants in the show interfaces output, combined with the interface being in err-disabled state, indicates a duplex mismatch. The show controllers output reveals that auto-negotiation is enabled, but the switch is operating at full-duplex while the link partner advertises only full-duplex capability. However, the presence of runts and giants suggests that one side is misconfigured for half-duplex, causing collisions and frame errors. The most common cause is a misconfigured duplex setting on the remote device, leading to a duplex mismatch. This mismatch causes the interface to go err-disabled due to excessive errors.
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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