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Network Infrastructure and Connectivity →hardTroubleshootingObjective-mapped

CCNA Network Infrastructure and Connectivity Practice Question

This 200-301 practice question tests your understanding of network infrastructure and connectivity. 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.

Network Topology
G0/110.0.0.5/30G0/110.0.0.6/30linkR1R2

You are connected to R1. The network administrator reports that the link between R1 and R2 is flapping and performance is poor. Examine the provided show interface output on R1, identify the root cause of the issue, and apply the necessary configuration fix to resolve the problem permanently.

Question 1hardTroubleshooting
Full question →

Exhibit

R1# show interfaces gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
  Hardware is ISR4331-2x1GE, address is aabb.cc00.0100 (bia aabb.cc00.0100)
  Internet address is 10.0.0.1/30
  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)
  Full-duplex, 100Mb/s, media type is RJ45
  output flow-control is unsupported, input 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 00:10:00
  Input queue: 0/375/0/0 (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
     120 packets input, 9600 bytes, 0 no buffer
     Received 0 broadcasts (0 IP 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
     120 packets output, 9600 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

R1# show interfaces gigabitEthernet 0/1
GigabitEthernet0/1 is up, line protocol is down
  Hardware is ISR4331-2x1GE, address is aabb.cc00.0101 (bia aabb.cc00.0101)
  Internet address is 10.0.0.5/30
  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 RJ45
  output flow-control is unsupported, input 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 00:10:00
  Input queue: 0/375/0/0 (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
     120 packets input, 9600 bytes, 0 no buffer
     Received 0 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     150 input errors, 150 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     120 packets output, 9600 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

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 the interface with the 'duplex full' command.

The show interface output on R1 indicates the interface is operating in half-duplex with high CRC errors and input errors, classic symptoms of a duplex mismatch when the remote side (R2) is set to full-duplex. The root cause is that R1’s duplex setting does not match R2’s, causing collisions and flapping. Configuring 'duplex full' on R1’s GigabitEthernet0/1 forces full-duplex, which resolves the mismatch if the remote side is already forced full. Option B (speed 100) only configures speed—it does not change duplex, so the mismatch persists. Option C (no shutdown) is irrelevant because the interface is administratively up (the issue is operational). Option D (duplex auto) would set R1 to autonegotiate, but if R2 is forced full, autonegotiation fails and defaults to half-duplex, recreating the mismatch. Therefore, only 'duplex full' permanently fixes the issue.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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 with the 'duplex full' command.

    Why this is correct

    This is correct because the show interface output indicates a duplex mismatch: R1 is set to half-duplex while the remote side is likely full-duplex, causing CRC errors and flapping. Setting duplex to full on R1 resolves the mismatch and stabilizes the link.

    Related concept

    Read the scenario before looking for a memorised answer.

  • ✗

    Configure the interface with the 'speed 100' command.

    Why it's wrong here

    This is incorrect because the issue is a duplex mismatch, not a speed mismatch. While speed should match on both ends, the primary problem is duplex; setting speed alone does not fix the duplex mismatch.

  • ✗

    Configure the interface with the 'no shutdown' command.

    Why it's wrong here

    This is incorrect because the interface is administratively up (not shutdown). The problem is a duplex mismatch, not an administrative shutdown.

  • ✗

    Configure the interface with the 'duplex auto' command.

    Why it's wrong here

    This is incorrect because setting duplex to auto on a manually configured full-duplex link can cause a mismatch. The remote side is likely set to full-duplex, so R1 must also be set to full-duplex manually to match.

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 the interface with the 'duplex full' command.Correct answer▾

Why this is correct

This is correct because the show interface output indicates a duplex mismatch: R1 is set to half-duplex while the remote side is likely full-duplex, causing CRC errors and flapping. Setting duplex to full on R1 resolves the mismatch and stabilizes the link.

✗Configure the interface with the 'speed 100' command.Wrong answer — click to see why▾

Why this is wrong here

The specific factual error is that speed mismatch is not the root cause; duplex mismatch is indicated by CRC errors and flapping.

Why candidates choose this

Candidates might think speed mismatch causes flapping, but duplex mismatch is more common and produces similar symptoms.

✗Configure the interface with the 'no shutdown' command.Wrong answer — click to see why▾

Why this is wrong here

The specific factual error is that 'no shutdown' only brings an interface up administratively; it does not fix duplex or speed issues.

Why candidates choose this

Candidates might assume the interface is down due to being shutdown, but the output shows 'line protocol is down' due to duplex mismatch, not administrative down.

✗Configure the interface with the 'duplex auto' command.Wrong answer — click to see why▾

Why this is wrong here

The specific factual error is that auto-negotiation can fail if one side is manually set; the fix is to manually set both sides to the same duplex.

Why candidates choose this

Candidates might think auto-negotiation always resolves issues, but in this scenario, the remote side is manually set to full-duplex, so auto would cause a mismatch.

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: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

KKey Concepts to Remember

  • Read the scenario before looking for a memorised answer.
  • Find the constraint that changes the correct option.
  • Eliminate answers that are true in general but not in this case.
  • Use explanations to understand the rule behind the answer.

TExam Day Tips

  • Underline the problem statement mentally.
  • Watch for words such as best, first, most likely and least administrative effort.
  • Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A practitioner preparing for the 200-301 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

What to study next

Got this wrong? Here's your next step.

Identify which 200-301 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this 200-301 question test?

Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: Configure the interface with the 'duplex full' command. — The show interface output on R1 indicates the interface is operating in half-duplex with high CRC errors and input errors, classic symptoms of a duplex mismatch when the remote side (R2) is set to full-duplex. The root cause is that R1’s duplex setting does not match R2’s, causing collisions and flapping. Configuring 'duplex full' on R1’s GigabitEthernet0/1 forces full-duplex, which resolves the mismatch if the remote side is already forced full. Option B (speed 100) only configures speed—it does not change duplex, so the mismatch persists. Option C (no shutdown) is irrelevant because the interface is administratively up (the issue is operational). Option D (duplex auto) would set R1 to autonegotiate, but if R2 is forced full, autonegotiation fails and defaults to half-duplex, recreating the mismatch. Therefore, only 'duplex full' permanently fixes the issue.

What should I do if I get this 200-301 question wrong?

Identify which 200-301 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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Last reviewed: Jun 6, 2026

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