Question 1,714 of 1,389
CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
R1# show interfaces GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up (connected)
Hardware is iGbE, address is 0011.2233.4455 (bia 0011.2233.4455)
Internet address is 192.168.100.1/24
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)
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 00:00:01, 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, 2 packets/sec
5 minute output rate 5000 bits/sec, 3 packets/sec
52345 packets input, 10243456 bytes, 0 no buffer
Received 0 broadcasts (0 multicasts)
0 runts, 0 giants, 0 throttles
5231 input errors, 5200 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
245678 packets output, 54839203 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 outRefer to the exhibit. A network engineer is investigating intermittent connectivity complaints on a gigabit uplink between two distribution switches. The engineer runs the show interfaces GigabitEthernet0/0 command on one of the switches. Based on the output, what is the most likely cause of the errors?
⚠ Common exam trap
Cisco often tests the distinction between CRC errors (physical layer) and late collisions (duplex mismatch), so the trap here is that candidates see errors and assume a duplex mismatch without checking for the specific error types like late collisions or alignment errors.
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
✓
A damaged or faulty cable is causing excessive CRC errors.
The output shows a high number of CRC errors and runts, which typically indicate a Layer 1 physical-layer issue such as a damaged or faulty cable. CRC errors occur when frames fail the cyclic redundancy check due to signal degradation, noise, or physical damage to the cabling. On a gigabit uplink, this is the most likely cause of intermittent connectivity.
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 interface is configured with an incorrect encapsulation type.
Why it's wrong here
The output shows 'Encapsulation ARPA,' Cisco's designation for standard Ethernet II. If the interface were configured with an incompatible encapsulation (such as SNAP, 802.1Q, or a proprietary type), it would typically fail to establish adjacency or pass frames at all—not silently corrupt the FCS. CRC errors are computed after the receiver has clocked in the bits, so they indicate physical-layer bit corruption from noise, attenuation, or a bad cable, not a logical framing mismatch.
- ✓
A damaged or faulty cable is causing excessive CRC errors.
Why this is correct
The exhibit displays 5200 CRC errors (more than 5000) and 5231 input errors. High CRC counts directly indicate that received frames are being corrupted by physical layer issues such as a damaged cable, loose connector, or EMI on the copper segment.
- ✗
A duplex mismatch exists between the connected devices.
Why it's wrong here
A duplex mismatch normally causes late collisions, runts, and excessive deferrals on the half-duplex side, while the full-duplex side may see intermittent CRC errors from frames fragmented by collisions. This interface is running full-duplex and explicitly reports zero collisions, late collisions, runts, or giants. The absence of these classic mismatch symptoms makes a duplex mismatch an unlikely cause of the steady 5,200 CRC errors.
- ✗
The interface is assigned to the wrong VLAN.
Why it's wrong here
Assigning the interface to the wrong VLAN would place it in an incorrect broadcast domain, so user traffic would be logically blocked or misdelivered, but the link's physical-layer behavior would remain unchanged. VLAN classification occurs after the frame's Frame Check Sequence is validated, because the 802.1Q header is part of the FCS calculation. A wrong VLAN therefore cannot increase the CRC error counter; those errors are caused by corrupted bits on the wire, not by forwarding table membership.
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.
✓A damaged or faulty cable is causing excessive CRC errors.Correct answer▾
Why this is correct
The exhibit displays 5200 CRC errors (more than 5000) and 5231 input errors. High CRC counts directly indicate that received frames are being corrupted by physical layer issues such as a damaged cable, loose connector, or EMI on the copper segment.
✗The interface is configured with an incorrect encapsulation type.Wrong answer — click to see why▾
Why this is wrong here
Encapsulation problems cause protocol failures, not corrupted frames with CRC errors.
✗A duplex mismatch exists between the connected devices.Wrong answer — click to see why▾
Why this is wrong here
Candidates often mistake high CRC counts for duplex issues. The absence of collision-related counters rules out a duplex mismatch.
✗The interface is assigned to the wrong VLAN.Wrong answer — click to see why▾
Why this is wrong here
A wrong VLAN does not generate CRC errors on the physical interface.
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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Last reviewed: Jun 25, 2026
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