Question 379 of 1,819
Network Infrastructure and ConnectivityhardTroubleshootingObjective-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.

Exhibit

R1# show interfaces gigabitethernet 0/0
GigabitEthernet0/0 is up, line protocol is down
  Hardware is GigabitEthernet, address is aabb.cc00.0100 (bia aabb.cc00.0100)
  Description: Link to SW1
  Internet address is 192.0.2.1/30
  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)
  Auto-duplex, Auto-speed
  input errors: 0 CRC, 0 frame, 0 overrun, 0 ignored
  output errors: 0 collisions, 0 interface resets
  unknown protocol drops: 0
  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
     0 packets input, 0 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
     0 input packets with dribble condition detected
     0 packets output, 0 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

R1# show interfaces gigabitethernet 0/0 transceiver
GigabitEthernet0/0: SFP is not present

R1# show running-config interface gigabitethernet 0/0
Building configuration...

Current configuration : 60 bytes
!
interface GigabitEthernet0/0
 description Link to SW1
 ip address 192.0.2.1 255.255.255.252
 no shutdown
end

R1# show ip interface brief
Interface                  IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0         192.0.2.1       YES manual up                    down
GigabitEthernet0/1         unassigned      YES unset  administratively down down

You are connected to R1. Configure the G0/0 interface to match the speed (100 Mbps) and duplex (full) of the connected switch port, then diagnose and fix an auto-negotiation failure that has caused excessive CRC errors. Finally, select and replace the SFP module on G0/0 with one that supports a 5 km fiber link, using the correct cable type.

Question 1hardTroubleshooting
Full question →

Exhibit

R1# show interfaces gigabitethernet 0/0
GigabitEthernet0/0 is up, line protocol is down
  Hardware is GigabitEthernet, address is aabb.cc00.0100 (bia aabb.cc00.0100)
  Description: Link to SW1
  Internet address is 192.0.2.1/30
  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)
  Auto-duplex, Auto-speed
  input errors: 0 CRC, 0 frame, 0 overrun, 0 ignored
  output errors: 0 collisions, 0 interface resets
  unknown protocol drops: 0
  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
     0 packets input, 0 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
     0 input packets with dribble condition detected
     0 packets output, 0 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

R1# show interfaces gigabitethernet 0/0 transceiver
GigabitEthernet0/0: SFP is not present

R1# show running-config interface gigabitethernet 0/0
Building configuration...

Current configuration : 60 bytes
!
interface GigabitEthernet0/0
 description Link to SW1
 ip address 192.0.2.1 255.255.255.252
 no shutdown
end

R1# show ip interface brief
Interface                  IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0         192.0.2.1       YES manual up                    down
GigabitEthernet0/1         unassigned      YES unset  administratively down down

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

Set speed 100, duplex full; replace SFP with 1000BASE-LX; use single-mode fiber (SMF).

The interface is up but line protocol is down, indicating a layer 1 issue. The switch port is manually set to 100 Mbps/full duplex, but R1 is using auto-negotiation, causing a mismatch. First, set the speed to 100 and duplex to full on G0/0. The CRC errors may appear later if cabling is faulty; for a 5 km fiber link, you need a 1000BASE-LX SFP (single-mode) and single-mode fiber (SMF). Verify the SFP is recognized and the link comes up.

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.

  • Set speed 100, duplex full; replace SFP with 1000BASE-LX; use single-mode fiber (SMF).

    Why this is correct

    This is correct because the switch port is manually set to 100 Mbps/full duplex, so R1 must match those settings to avoid a mismatch. CRC errors indicate a duplex mismatch or faulty cabling; after fixing the mismatch, replacing the SFP with 1000BASE-LX (which supports up to 5 km over single-mode fiber) and using SMF resolves the distance requirement.

    Related concept

    Read the scenario before looking for a memorised answer.

  • Set speed 100, duplex half; replace SFP with 1000BASE-SX; use multimode fiber (MMF).

    Why it's wrong here

    This is incorrect because the switch port is set to full duplex, so setting duplex half on R1 would cause a duplex mismatch, leading to CRC errors. Additionally, 1000BASE-SX supports only up to 550 m over MMF, not 5 km.

  • Set speed 1000, duplex full; replace SFP with 1000BASE-LX; use single-mode fiber (SMF).

    Why it's wrong here

    This is incorrect because the switch port is set to 100 Mbps, not 1000 Mbps. Setting speed 1000 would cause a speed mismatch, and the interface would not come up. The SFP and fiber type are correct for 5 km, but the speed must match the switch.

  • Set no speed, no duplex (auto); replace SFP with 1000BASE-LX; use single-mode fiber (SMF).

    Why it's wrong here

    This is incorrect because leaving speed and duplex on auto will not match the switch's manual settings, causing a mismatch. The switch port is manually set to 100/full, so auto-negotiation on R1 will fail, leading to a duplex mismatch and CRC errors.

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.

Set speed 100, duplex full; replace SFP with 1000BASE-LX; use single-mode fiber (SMF).Correct answer

Why this is correct

This is correct because the switch port is manually set to 100 Mbps/full duplex, so R1 must match those settings to avoid a mismatch. CRC errors indicate a duplex mismatch or faulty cabling; after fixing the mismatch, replacing the SFP with 1000BASE-LX (which supports up to 5 km over single-mode fiber) and using SMF resolves the distance requirement.

Set speed 100, duplex half; replace SFP with 1000BASE-SX; use multimode fiber (MMF).Wrong answer — click to see why

Why this is wrong here

The specific factual error: Duplex must match the switch (full), and 1000BASE-SX cannot reach 5 km.

Why candidates choose this

Candidates might think half duplex is acceptable or confuse SX with LX, and MMF is common for shorter distances.

Set speed 1000, duplex full; replace SFP with 1000BASE-LX; use single-mode fiber (SMF).Wrong answer — click to see why

Why this is wrong here

The specific factual error: Speed must match the switch port's configured speed (100 Mbps), not auto-negotiate to 1000 Mbps.

Why candidates choose this

Candidates may assume that a 1000BASE-LX SFP requires gigabit speed, but the switch port is manually set to 100 Mbps, so R1 must match that.

Set no speed, no duplex (auto); replace SFP with 1000BASE-LX; use single-mode fiber (SMF).Wrong answer — click to see why

Why this is wrong here

The specific factual error: Auto-negotiation cannot match a manually configured port; both sides must be manually set or both use auto-negotiation.

Why candidates choose this

Candidates may think auto-negotiation will automatically adjust, but it fails when one side is manually configured.

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.

Related practice questions

Related 200-301 practice-question pages

Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.

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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: Set speed 100, duplex full; replace SFP with 1000BASE-LX; use single-mode fiber (SMF). — The interface is up but line protocol is down, indicating a layer 1 issue. The switch port is manually set to 100 Mbps/full duplex, but R1 is using auto-negotiation, causing a mismatch. First, set the speed to 100 and duplex to full on G0/0. The CRC errors may appear later if cabling is faulty; for a 5 km fiber link, you need a 1000BASE-LX SFP (single-mode) and single-mode fiber (SMF). Verify the SFP is recognized and the link comes up.

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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This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.