Question 582 of 1,020

Network Configuration Concepts →mediumMultiple ChoiceObjective-mapped

Why a Gigabit Switch Port Shows 100 Mbps: Client NIC Limitation

This 220-1201 practice question tests your understanding of network configuration concepts. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.

During a network upgrade, a technician replaces an old 10/100 Mbps switch with a new gigabit switch. After the change, several users report that their internet connection is slow. The technician checks the switch and sees that some ports show 100 Mbps instead of 1000 Mbps. What is the most likely cause?

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.

A
The switch ports are configured for half-duplex
Why wrong: Half-duplex would cause collisions and poor performance, but the symptom of showing 100 Mbps indicates a speed mismatch, not duplex.
B
The Ethernet cables are Cat5 instead of Cat5e or Cat6
Why wrong: Cat5 can support gigabit at shorter distances, but the issue is that the switch is negotiating at 100 Mbps, which is more likely due to the client NIC.
C
The connected devices have 10/100 Mbps NICs
If the client NIC only supports 100 Mbps, the switch will negotiate to that speed, resulting in slower performance than gigabit.
D
The switch has a faulty power supply causing reduced performance
Why wrong: A faulty power supply would likely cause intermittent connectivity or port failures, not a consistent speed negotiation to 100 Mbps.

Full breakdown with real-world context →

Quick Answer

The answer is that the connected devices have 10/100 Mbps NICs, which forces the gigabit switch port to show 100 Mbps due to auto-negotiation. When a client’s network interface card only supports 100 Mbps, the switch and NIC exchange capabilities and automatically settle on the highest mutually supported speed, which in this case is 100 Mbps rather than 1000 Mbps. On the CompTIA A+ Core 1 220-1201 exam, this scenario tests your understanding of Ethernet speed negotiation and the principle that a switch cannot exceed the capability of the connected client hardware. A common trap is to suspect a faulty cable or switch configuration, but the real bottleneck is the client NIC limitation. Remember the memory tip: “The switch can only go as fast as the slowest NIC in the chain.”

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

✓

The connected devices have 10/100 Mbps NICs

Option C is correct because the most likely cause of the switch ports operating at 100 Mbps instead of 1000 Mbps is that the connected devices have 10/100 Mbps NICs. Gigabit Ethernet requires both the switch port and the connected device to support 1000BASE-T; if the NIC only supports 10/100, the switch will auto-negotiate down to the highest common speed, which is 100 Mbps. This explains why only some users experience slowness—those with older NICs are limited to 100 Mbps while others with gigabit-capable NICs get full speed.

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.

✗
The switch ports are configured for half-duplex
Why it's wrong here
Half-duplex would cause collisions and poor performance, but the symptom of showing 100 Mbps indicates a speed mismatch, not duplex.
✗
The Ethernet cables are Cat5 instead of Cat5e or Cat6
Why it's wrong here
Cat5 can support gigabit at shorter distances, but the issue is that the switch is negotiating at 100 Mbps, which is more likely due to the client NIC.
✓
The connected devices have 10/100 Mbps NICs
Why this is correct
If the client NIC only supports 100 Mbps, the switch will negotiate to that speed, resulting in slower performance than gigabit.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The switch has a faulty power supply causing reduced performance
Why it's wrong here
A faulty power supply would likely cause intermittent connectivity or port failures, not a consistent speed negotiation to 100 Mbps.

Common exam traps

Common exam trap: answer the scenario, not the keyword

A common misconception is that cable category (Cat5 vs. Cat5e) is the primary cause of speed negotiation failures, but in reality, the most common cause is the endpoint NIC's capability, as auto-negotiation will still attempt gigabit over Cat5 and only fall back if the cable fails or the NIC doesn't support it.

Trap categories for this question

Command / output trap
Half-duplex would cause collisions and poor performance, but the symptom of showing 100 Mbps indicates a speed mismatch, not duplex.

Detailed technical explanation

How to think about this question

Auto-negotiation for 1000BASE-T uses a base page and next page exchange to advertise capabilities, including speed and duplex; if the NIC does not advertise 1000 Mbps, the switch will fall back to 100BASE-TX. In practice, many older devices (e.g., printers, IP phones, or legacy desktops) still use 10/100 NICs, and this is the most common reason for a gigabit switch to show 100 Mbps on certain ports. A real-world scenario is upgrading a network switch but not the endpoint devices, leading to mixed speeds that can cause confusion if the technician assumes all devices are gigabit-capable.

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.

TExam Day Tips

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 220-1201 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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 220-1201 question test?

Network Configuration Concepts — This question tests Network Configuration Concepts — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The connected devices have 10/100 Mbps NICs — Option C is correct because the most likely cause of the switch ports operating at 100 Mbps instead of 1000 Mbps is that the connected devices have 10/100 Mbps NICs. Gigabit Ethernet requires both the switch port and the connected device to support 1000BASE-T; if the NIC only supports 10/100, the switch will auto-negotiate down to the highest common speed, which is 100 Mbps. This explains why only some users experience slowness—those with older NICs are limited to 100 Mbps while others with gigabit-capable NICs get full speed.

What should I do if I get this 220-1201 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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?

Read the scenario before looking for a memorised answer.

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