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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
G0/010.0.0.1/30G0/010.0.0.2/30linkR1R2

You are connected to R1. The network team reports intermittent connectivity between R1 and R2. Examine the following `show interface` output from R1 to identify the root cause of the issue. Then apply the necessary configuration commands on R1 to resolve the problem.

GigabitEthernet0/0 is up, line protocol is up 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 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 100 packets input, 8000 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 100 input errors, 100 CRC, 0 frame, 0 overrun, 0 ignored 0 output errors, 0 collisions, 0 interface resets 0 output buffer failures, 0 output buffers swapped out

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

  • Clue: "never"

    Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.

Question 1hardTroubleshooting
Full question →

Exhibit

R1# show interfaces GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
  Hardware is CN Gigabit Ethernet, address is 0000.0c12.3456 (bia 0000.0c12.3456)
  Internet address is 10.0.0.1/30
  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, 100Mb/s, media type is RJ45
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:05, 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 2000 bits/sec, 3 packets/sec
     1500 packets input, 120000 bytes, 0 no buffer
     Received 50 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     100 input errors, 100 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     1500 packets output, 130000 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

✓

Enter interface configuration mode for the problematic interface and issue the 'no speed' and 'no duplex' commands to revert to auto-negotiation.

The show interface output reveals 100 CRC errors and 100 input errors, indicating a physical layer problem. Additionally, the interface is manually configured for full-duplex and 100 Mb/s. CRC errors often point to a duplex mismatch or faulty cable. Since R1 is set to full-duplex, if R2 is set to auto-negotiation (or half-duplex), a mismatch occurs. The fix is to set both sides to auto-negotiation by removing manual speed and duplex settings. On R1, enter interface configuration mode and issue 'no speed' and 'no duplex' to revert to auto-negotiation, then verify with 'show interfaces' that the interface negotiates to full-duplex and errors stop incrementing.

Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

  • ✓

    Enter interface configuration mode for the problematic interface and issue the 'no speed' and 'no duplex' commands to revert to auto-negotiation.

    Why this is correct

    This is correct because CRC errors and input errors often indicate a duplex mismatch. Manually setting speed and duplex on one side while the other uses auto-negotiation can cause such errors. Removing manual settings allows both sides to auto-negotiate, resolving the mismatch.

    Clue confirmation

    The clue word "never" in the question point toward this answer.

    Related concept

    Authentication checks who the user is.

  • ✗

    Enter interface configuration mode and issue the 'speed 100' and 'duplex full' commands to ensure both sides are set to the same values.

    Why it's wrong here

    This is incorrect because the interface is already manually set to speed 100 and duplex full, yet errors persist. Reapplying the same settings does not address the root cause; the problem is likely a mismatch with the remote side.

  • ✗

    Enter interface configuration mode and issue the 'no shutdown' command to bring the interface up.

    Why it's wrong here

    This is incorrect because the interface is already up (as indicated by the show interface output showing line protocol is up). The issue is not administrative shutdown but physical layer errors.

  • ✗

    Enter interface configuration mode and issue the 'duplex half' command to match the remote side's duplex setting.

    Why it's wrong here

    This is incorrect because setting duplex to half on R1 would create a mismatch if R2 is using full-duplex (which is typical for 100 Mb/s links). The correct approach is to auto-negotiate, not force half-duplex.

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.

✓Enter interface configuration mode for the problematic interface and issue the 'no speed' and 'no duplex' commands to revert to auto-negotiation.Correct answer▾

Why this is correct

This is correct because CRC errors and input errors often indicate a duplex mismatch. Manually setting speed and duplex on one side while the other uses auto-negotiation can cause such errors. Removing manual settings allows both sides to auto-negotiate, resolving the mismatch.

✗Enter interface configuration mode and issue the 'speed 100' and 'duplex full' commands to ensure both sides are set to the same values.Wrong answer — click to see why▾

Why this is wrong here

The specific factual error is that reapplying the same manual settings does not resolve a duplex mismatch; it may even perpetuate the mismatch if the remote side is set to auto-negotiation.

Why candidates choose this

Candidates might think that ensuring both sides have the same manual settings will fix the issue, but they overlook that the remote side might be using auto-negotiation, which requires both sides to auto-negotiate.

✗Enter interface configuration mode and issue the 'no shutdown' command to bring the interface up.Wrong answer — click to see why▾

Why this is wrong here

The specific factual error is that 'no shutdown' is used to enable an interface that is administratively down, but the interface is already up.

Why candidates choose this

Candidates might assume that intermittent connectivity is due to the interface being down, but the show interface output would show 'administratively down' if that were the case.

✗Enter interface configuration mode and issue the 'duplex half' command to match the remote side's duplex setting.Wrong answer — click to see why▾

Why this is wrong here

The specific factual error is that forcing half-duplex is unlikely to resolve the issue; it may worsen performance and still cause errors if the remote side is full-duplex.

Why candidates choose this

Candidates might think that since CRC errors indicate a duplex mismatch, setting both sides to half-duplex would fix it, but they ignore that the remote side might be set to full-duplex or auto-negotiation.

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: authentication is not authorization

Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.

Trap categories for this question

  • Command / output trap

    This is incorrect because the interface is already up (as indicated by the show interface output showing line protocol is up). The issue is not administrative shutdown but physical layer errors.

Detailed technical explanation

How to think about this question

This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.

KKey Concepts to Remember

  • Authentication checks who the user is.
  • Authorization controls what the user is allowed to do after login.
  • Privilege levels affect access to EXEC and configuration commands.
  • AAA, TACACS+ and RADIUS can separate login success from command access.

TExam Day Tips

  • Do not assume successful login means full administrative access.
  • Look for words such as cannot enter configuration mode, privilege level, authorization or command access.
  • Separate login problems from permission problems before choosing the answer.

Key takeaway

Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.

Real-world example

How this comes up in practice

A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

What to study next

Got this wrong? Here's your next step.

Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 200-301 questions on access control and AAA configuration.

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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 — Authentication checks who the user is..

What is the correct answer to this question?

The correct answer is: Enter interface configuration mode for the problematic interface and issue the 'no speed' and 'no duplex' commands to revert to auto-negotiation. — The show interface output reveals 100 CRC errors and 100 input errors, indicating a physical layer problem. Additionally, the interface is manually configured for full-duplex and 100 Mb/s. CRC errors often point to a duplex mismatch or faulty cable. Since R1 is set to full-duplex, if R2 is set to auto-negotiation (or half-duplex), a mismatch occurs. The fix is to set both sides to auto-negotiation by removing manual speed and duplex settings. On R1, enter interface configuration mode and issue 'no speed' and 'no duplex' to revert to auto-negotiation, then verify with 'show interfaces' that the interface negotiates to full-duplex and errors stop incrementing.

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

Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 200-301 questions on access control and AAA configuration.

Are there clue words in this question I should notice?

Yes — watch for: "never". Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.

What is the key concept behind this question?

Authentication checks who the user is.

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Last reviewed: Jun 6, 2026

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