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Question 1,269 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: A network administrator is troubleshooting…
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)
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 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 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
12345 packets input, 1234567 bytes, 0 no buffer
Received 123 broadcasts (0 multicast)
0 runts, 0 giants, 0 throttles
1000 CRC errors, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
23456 packets output, 2345678 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
SwitchB# show interfaces gigabitethernet 0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is dddd.eeee.ffff (bia dddd.eeee.ffff)
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Half-duplex, 100Mb/s, 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 00:00:01, output 00:00:01, 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 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
54321 packets input, 543210 bytes, 0 no buffer
Received 321 broadcasts (0 multicast)
0 runts, 0 giants, 0 throttles
0 CRC errors, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
65432 packets output, 654321 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 administrator is troubleshooting connectivity between two directly connected Cisco switches. Hosts on VLAN 10 connected to Switch A cannot ping the default gateway on Switch B. The interface on Switch A shows 'up/up' but there are excessive CRC errors and runts. The administrator checks the interface configuration on both switches. 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
✓
Configure both interfaces with the same duplex and speed settings (e.g., full-duplex and 1000 Mbps).
The root cause is a duplex mismatch. Switch A is configured for full-duplex at 1000 Mbps, while Switch B is operating at half-duplex at 100 Mbps. This mismatch causes CRC errors on the full-duplex side due to collisions and frame corruption. The CRC errors indicate that frames are being corrupted, which is a classic symptom of duplex mismatch. The correct solution is to set both interfaces to the same duplex setting, typically full-duplex, and ensure speed matches. The other options are incorrect because the interface is up/up, so a faulty cable or VLAN mismatch would not produce CRC errors, and late collisions would appear if the cable was too long, which is not indicated.
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 faulty Ethernet cable.
Why it's wrong here
A faulty cable could cause CRC errors, but it would also typically cause intermittent connectivity or interface flapping, and the speed mismatch between the two switches suggests a configuration issue rather than a physical cable problem.
- ✓
Configure both interfaces with the same duplex and speed settings (e.g., full-duplex and 1000 Mbps).
Why this is correct
This directly resolves the duplex mismatch. Switch A shows full-duplex/1000 Mbps, while Switch B shows half-duplex/100 Mbps. Setting both to full-duplex and 1000 Mbps eliminates the CRC errors caused by the mismatch.
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.
- ✗
Disable spanning tree on both interfaces to prevent loop prevention from blocking traffic.
Why it's wrong here
Spanning tree does not cause CRC errors or duplex mismatch issues. Disabling it would risk loops and is not relevant to the symptoms shown.
- ✗
Assign the interfaces to the same VLAN to ensure Layer 2 connectivity.
Why it's wrong here
VLAN mismatch would cause the line protocol to be up but no traffic to pass for that VLAN, but it would not produce CRC errors on the interface.
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.
✓Configure both interfaces with the same duplex and speed settings (e.g., full-duplex and 1000 Mbps).Correct answer▾
Why this is correct
This directly resolves the duplex mismatch. Switch A shows full-duplex/1000 Mbps, while Switch B shows half-duplex/100 Mbps. Setting both to full-duplex and 1000 Mbps eliminates the CRC errors caused by the mismatch.
✗Replace the faulty Ethernet cable.Wrong answer — click to see why▾
Why this is wrong here
The exhibit shows both switches have different speed and duplex settings, indicating a configuration mismatch rather than a cable fault.
✗Disable spanning tree on both interfaces to prevent loop prevention from blocking traffic.Wrong answer — click to see why▾
Why this is wrong here
The interface is up/up and the errors are CRC, which are not related to spanning tree operation.
✗Assign the interfaces to the same VLAN to ensure Layer 2 connectivity.Wrong answer — click to see why▾
Why this is wrong here
CRC errors indicate physical or duplex issues, not VLAN misconfiguration.
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
Spanning tree does not cause CRC errors or duplex mismatch issues. Disabling it would risk loops and is not relevant to the symptoms shown.
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: Configure both interfaces with the same duplex and speed settings (e.g., full-duplex and 1000 Mbps). — The root cause is a duplex mismatch. Switch A is configured for full-duplex at 1000 Mbps, while Switch B is operating at half-duplex at 100 Mbps. This mismatch causes CRC errors on the full-duplex side due to collisions and frame corruption. The CRC errors indicate that frames are being corrupted, which is a classic symptom of duplex mismatch. The correct solution is to set both interfaces to the same duplex setting, typically full-duplex, and ensure speed matches. The other options are incorrect because the interface is up/up, so a faulty cable or VLAN mismatch would not produce CRC errors, and late collisions would appear if the cable was too long, which is not indicated.
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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