CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
SwitchA# show interfaces gigabitethernet 0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
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 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, 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 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
12345 packets input, 1234567 bytes, 0 no buffer
Received 123 broadcasts (0 multicast)
0 runts, 0 giants, 0 throttles
1000 CRC errors, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
23456 packets output, 2345678 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
SwitchB# show interfaces gigabitethernet 0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is dddd.eeee.ffff (bia dddd.eeee.ffff)
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)
Half-duplex, 100Mb/s, media type is 10/100/1000BaseTX
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 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
54321 packets input, 543210 bytes, 0 no buffer
Received 321 broadcasts (0 multicast)
0 runts, 0 giants, 0 throttles
0 CRC errors, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
65432 packets output, 654321 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 outA network administrator is troubleshooting connectivity between two directly connected Cisco switches. Hosts on VLAN 10 connected to Switch A cannot ping the default gateway on Switch B. The interface on Switch A shows 'up/up' but there are excessive CRC errors and runts. The administrator checks the interface configuration on both switches. What is the most likely cause of the issue?
⚠ Common exam trap
Cisco often tests the concept that 'up/up' does not guarantee error-free communication, and candidates mistakenly focus on cable replacement or VLAN misconfiguration instead of recognizing CRC errors and runts as classic symptoms of a duplex mismatch.
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
✓
Mismatched duplex and speed settings between the interfaces.
The presence of excessive CRC errors and runts on an interface that is 'up/up' strongly indicates a Layer 1 or Layer 2 duplex mismatch. When one switch is set to full-duplex and the other to half-duplex (or auto-negotiation fails), the half-duplex side will detect collisions and retransmit, while the full-duplex side will not, leading to frame corruption (CRC errors) and truncated frames (runts). Configuring both interfaces with the same duplex and speed settings (e.g., full-duplex and 1000 Mbps) resolves this mismatch, restoring proper connectivity for VLAN 10 traffic to the default gateway.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Replace the faulty Ethernet cable.
Why it's wrong here
A faulty cable could cause CRC errors, but it would also typically cause intermittent connectivity or interface flapping, and the speed mismatch between the two switches suggests a configuration issue rather than a physical cable problem.
- ✓
Mismatched duplex and speed settings between the interfaces.
Why this is correct
With one switch operating at full-duplex and 1000 Mbps while the other operates at half-duplex and 100 Mbps, the two interfaces cannot coordinate frame timing properly. The full-duplex side transmits without carrier sensing, while the half-duplex side detects a collision, sends a jam signal, and drops the corrupted frame—producing CRC errors on the half-duplex interface. Forcing both interfaces to the same speed (1000 Mbps) and full-duplex mode eliminates the mismatch, allowing the error counters to stop incrementing.
- ✗
Disable spanning tree on both interfaces to prevent loop prevention from blocking traffic.
Why it's wrong here
Spanning Tree Protocol (STP) prevents Layer 2 forwarding loops by blocking redundant ports; it has no effect on a link's physical signaling, clocking, or duplex mode. CRC errors are caused by frame corruption from collisions, bad cabling, or electromagnetic interference—not by STP port states. Disabling STP on both interfaces would only create a risk of broadcast storms and MAC table instability, and it would not correct the fundamental speed/duplex negotiation failure between the two switches.
- ✗
Assign the interfaces to the same VLAN to ensure Layer 2 connectivity.
Why it's wrong here
A VLAN membership mismatch prevents traffic for that VLAN from crossing the link because VLAN tagging does not match, but it does not affect how the physical interface negotiates speed or duplex. CRC errors are counted at Layer 1/Layer 2 when the frame check sequence fails, which occurs independently of the VLAN ID assigned to the interface. Reassigning the interfaces to the same VLAN would not lower the CRC error count because the error is caused by the physical layer parameters, not by VLAN segmentation.
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.
✓Mismatched duplex and speed settings between the interfaces.Correct answer▾
Why this is correct
With one switch operating at full-duplex and 1000 Mbps while the other operates at half-duplex and 100 Mbps, the two interfaces cannot coordinate frame timing properly. The full-duplex side transmits without carrier sensing, while the half-duplex side detects a collision, sends a jam signal, and drops the corrupted frame—producing CRC errors on the half-duplex interface. Forcing both interfaces to the same speed (1000 Mbps) and full-duplex mode eliminates the mismatch, allowing the error counters to stop incrementing.
✗Replace the faulty Ethernet cable.Wrong answer — click to see why▾
Why this is wrong here
The exhibit shows both switches have different speed and duplex settings, indicating a configuration mismatch rather than a cable fault.
✗Disable spanning tree on both interfaces to prevent loop prevention from blocking traffic.Wrong answer — click to see why▾
Why this is wrong here
The interface is up/up and the errors are CRC, which are not related to spanning tree operation.
✗Assign the interfaces to the same VLAN to ensure Layer 2 connectivity.Wrong answer — click to see why▾
Why this is wrong here
CRC errors indicate physical or duplex issues, not VLAN misconfiguration.
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?”
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Related to this question
Learn chapter
ICMP and Ping
Key term
Ping
Ping is a network utility used to test whether a remote computer or device is reachable across an IP network and to measure the round-trip time of data packets.
Key term
CRC
CRC (Cyclic Redundancy Check) is an error-detecting code used to detect accidental changes to raw data in digital networks.
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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.