Duplex Mismatch Troubleshooting: Late Collisions and CRC Errors
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 outRefer 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?
Quick Answer
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.
⚠ Common exam trap
Cisco often tests duplex mismatch by showing output with late collisions and CRC errors, and the trap here is that candidates may blame the cable (Option B) or assume the switch port is half-duplex (Option A), when the real issue is the router interface operating in half-duplex against a full-duplex switch port.
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 router interface is operating in half-duplex while the switch port is full-duplex, causing a duplex mismatch.
The output shows the router interface is operating in half-duplex (as indicated by 'Half-duplex' in the show interfaces output), while the switch port is manually configured for full-duplex. This duplex mismatch causes the router to use CSMA/CD and detect collisions, while the switch transmits without collision detection, leading to late collisions, CRC errors, and intermittent connectivity. The correct fix is to set both sides to the same duplex setting, ideally full-duplex.
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).
- ✗
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?”
Go deeper
Related to this question
Learn chapter
Collision Domains and Broadcast Domains
Key term
CRC
CRC (Cyclic Redundancy Check) is an error-detecting code used to detect accidental changes to raw data in digital networks.
Key term
Cyclic Redundancy Check
A Cyclic Redundancy Check (CRC) is an error-detecting code used to verify that data transmitted over a network or stored on a device has not been corrupted.
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Same concept, more angles
1 more way this is tested on 200-301
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. R1# show interface GigabitEthernet0/0 GigabitEthernet0/0 is up, line protocol is up (connected) Hardware is Gigabit Ethernet, address is 0011.2233.4455 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, 1000Mb/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 00:00:01, 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 1000 bits/sec, 1 packets/sec 5 minute output rate 1000 bits/sec, 1 packets/sec 2453 packets input, 213456 bytes, 0 no buffer Received 500 broadcasts (0 multicasts) 0 runts, 0 giants, 0 throttles 312 input errors, 312 CRC, 0 frame, 0 overrun, 0 ignored 0 watchdog, 0 multicast, 0 pause input 3211 packets output, 345678 bytes, 0 underruns 0 output errors, 0 collisions, 0 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
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 B: The output shows the interface is operating at Full-duplex, 1000Mb/s, but is experiencing a high number of input errors and CRC errors. On Gigabit Ethernet, a duplex mismatch is not possible because 1000BASE-T operates in full-duplex mode only. The CRC and input errors point to physical layer problems, such as a faulty cable or connector, causing intermittent packet loss. Replacing the cable between R1 and R2 is the appropriate fix.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.