The answer is a duplex mismatch, specifically that the router interface is operating in half-duplex while the upstream switch port is set to full-duplex. This is the most likely cause because the show interfaces output reveals 'Half-duplex, 1000Mb/s' alongside 17 late collisions and 23 input CRC errors—classic symptoms of duplex mismatch troubleshooting scenarios where one side expects to transmit and receive simultaneously while the other cannot. On the CCNA 200-301 v2 exam, this question tests your ability to correlate interface statistics with configuration mismatches, often using a trap where a bad cable is blamed first; however, the cable is already verified as working. The presence of late collisions on a GigabitEthernet link is the definitive clue, as full-duplex links should never have collisions. Memory tip: Late collisions = duplex confusion; if you see them, check both sides for speed and duplex agreement.
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 GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
Hardware is iGbE, address is 00ab.cd12.3456 (bia 00ab.cd12.3456)
Internet address is 192.168.1.1/24
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)
Half-duplex, 1000Mb/s, media type is RJ45
input flow-control is off, output flow-control is off
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
12345 packets input, 987654 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
23 input errors, 23 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
5678 packets output, 345678 bytes, 0 underruns
0 output errors, 0 collisions, 17 late collisions, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out
Refer to the exhibit. A network engineer is troubleshooting intermittent connectivity between a branch router and the upstream switch. The switch port is manually configured for full-duplex, and the Ethernet cable has been tested and is working properly. The engineer runs the show interfaces GigabitEthernet0/0 command on the router and receives the output shown. Based on the output, what is the most likely cause of the problem?
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.
R1# show interfaces GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
Hardware is iGbE, address is 00ab.cd12.3456 (bia 00ab.cd12.3456)
Internet address is 192.168.1.1/24
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)
Half-duplex, 1000Mb/s, media type is RJ45
input flow-control is off, output flow-control is off
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
12345 packets input, 987654 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
23 input errors, 23 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
5678 packets output, 345678 bytes, 0 underruns
0 output errors, 0 collisions, 17 late collisions, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out
A
The switch port is configured for half-duplex instead of the expected full-duplex.
Why wrong: If the switch were also half-duplex, the link would operate without duplex mismatch symptoms; both sides would use CSMA/CD, and normal collisions (not necessarily late collisions) would occur. The scenario explicitly states the switch is set to full-duplex.
B
The cable is faulty, causing excessive late collisions and CRC errors.
Why wrong: Late collisions can be caused by excessive cable length or hardware faults, but the question states the cable has been tested and is working properly. Therefore, the cable is not the root cause.
C
The router interface is operating in half-duplex while the switch port is full-duplex, causing a duplex mismatch.
The output shows 'Half-duplex, 1000Mb/s' and reports 17 late collisions and 23 CRC errors. With the switch known to be full-duplex, this is a classic duplex mismatch scenario, where the full-duplex switch transmits without sensing the medium, while the half-duplex router interprets simultaneous traffic as collisions (many of them late because the switch may start transmitting after the router has already begun its frame).
D
The excessive input errors are a result of a broadcast storm on the network.
Why wrong: A broadcast storm would cause a high number of broadcast packets received, not CRC errors and late collisions. The exhibit shows zero broadcasts received and no abnormal queue drops. Broadcast storms do not generate late collisions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The router interface is operating in half-duplex while the switch port is full-duplex, causing a duplex mismatch.
The exhibit clearly displays 'Half-duplex, 1000Mb/s' in the interface characteristics, while the scenario states the upstream switch port is set to full-duplex. This duplex mismatch is confirmed by the presence of 17 late collisions and 23 input CRC errors, which are classic symptoms of one side operating full-duplex and the other half-duplex. A properly negotiated or statically matched full-duplex GigabitEthernet link would not exhibit late collisions.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 port is configured for half-duplex instead of the expected full-duplex.
Why it's wrong here
If the switch were also half-duplex, the link would operate without duplex mismatch symptoms; both sides would use CSMA/CD, and normal collisions (not necessarily late collisions) would occur. The scenario explicitly states the switch is set to full-duplex.
✗
The cable is faulty, causing excessive late collisions and CRC errors.
Why it's wrong here
Late collisions can be caused by excessive cable length or hardware faults, but the question states the cable has been tested and is working properly. Therefore, the cable is not the root cause.
✓
The router interface is operating in half-duplex while the switch port is full-duplex, causing a duplex mismatch.
Why this is correct
The output shows 'Half-duplex, 1000Mb/s' and reports 17 late collisions and 23 CRC errors. With the switch known to be full-duplex, this is a classic duplex mismatch scenario, where the full-duplex switch transmits without sensing the medium, while the half-duplex router interprets simultaneous traffic as collisions (many of them late because the switch may start transmitting after the router has already begun its frame).
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The excessive input errors are a result of a broadcast storm on the network.
Why it's wrong here
A broadcast storm would cause a high number of broadcast packets received, not CRC errors and late collisions. The exhibit shows zero broadcasts received and no abnormal queue drops. Broadcast storms do not generate late collisions.
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.
✓The router interface is operating in half-duplex while the switch port is full-duplex, causing a duplex mismatch.Correct answer▾
Why this is correct
The output shows 'Half-duplex, 1000Mb/s' and reports 17 late collisions and 23 CRC errors. With the switch known to be full-duplex, this is a classic duplex mismatch scenario, where the full-duplex switch transmits without sensing the medium, while the half-duplex router interprets simultaneous traffic as collisions (many of them late because the switch may start transmitting after the router has already begun its frame).
✗The switch port is configured for half-duplex instead of the expected full-duplex.Wrong answer — click to see why▾
Why this is wrong here
Users often think any collision indicator points to both sides being half-duplex, ignoring the known switch configuration.
✗The cable is faulty, causing excessive late collisions and CRC errors.Wrong answer — click to see why▾
Why this is wrong here
Late collisions are commonly associated with physical layer issues, leading candidates to assume a cable problem even when explicitly ruled out.
✗The excessive input errors are a result of a broadcast storm on the network.Wrong answer — click to see why▾
Why this is wrong here
Candidates see input errors and CRC and prematurely conclude a loop or broadcast storm, overlooking that the output shows no broadcast activity and contains late collisions specific to duplex issues.
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: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Trap categories for this question
Command / output trap
A broadcast storm would cause a high number of broadcast packets received, not CRC errors and late collisions. The exhibit shows zero broadcasts received and no abnormal queue drops. Broadcast storms do not generate late collisions.
Scenario analysis trap
If the switch were also half-duplex, the link would operate without duplex mismatch symptoms; both sides would use CSMA/CD, and normal collisions (not necessarily late collisions) would occur. The scenario explicitly states the switch is set to full-duplex.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 200-301 subnetting questions on CIDR, address ranges, and subnet selection.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The router interface is operating in half-duplex while the switch port is full-duplex, causing a duplex mismatch. — The exhibit clearly displays 'Half-duplex, 1000Mb/s' in the interface characteristics, while the scenario states the upstream switch port is set to full-duplex. This duplex mismatch is confirmed by the presence of 17 late collisions and 23 input CRC errors, which are classic symptoms of one side operating full-duplex and the other half-duplex. A properly negotiated or statically matched full-duplex GigabitEthernet link would not exhibit late collisions.
What should I do if I get this 200-301 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 200-301 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. You are troubleshooting connectivity between R1 and R2. The link is up but users report intermittent packet loss. Examine the provided show interface output on R1, identify the root cause, and apply the necessary fix to restore normal operation.
hard
✓ A.Configure 'duplex auto' and 'speed auto' on interface G0/0 of R1.
B.Replace the faulty cable between R1 and R2.
C.Increase the MTU size on interface G0/0 of R1.
D.Disable CDP on interface G0/0 of R1.
Why A: The interface shows 'input errors' (150) but zero CRC and zero frame errors. This combination, along with 'Full-duplex, 1000Mb/s' and the link being up/up, indicates the interface is manually set to full-duplex while the connected device (R2) is likely operating at half-duplex (duplex mismatch). Although CRC errors are zero, input errors can still occur due to collisions on a mismatched duplex link. The fix is to set the interface to auto-negotiate duplex and speed, or to manually set both sides to the same duplex setting. The recommended command is 'duplex auto' and 'speed auto' on both ends. In this scenario, we will configure R1's G0/0 for auto-negotiation.
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