CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
SwitchA# show interfaces gigabitethernet1/0/1
GigabitEthernet1/0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
Description: Fiber uplink to SwitchB
Internet address is 192.168.1.1/30
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, 1000Mbps, link type is auto, 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 1000 bits/sec, 2 packets/sec
5 minute output rate 1000 bits/sec, 2 packets/sec
1250 packets input, 125000 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
1250 input errors, 1250 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
1250 packets output, 125000 bytes, 0 underruns
0 output errors, 0 collisions, 1 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
SwitchA# show interfaces transceiver details gigabitethernet1/0/1
Transceiver Type: SFP
Product Name: SFP-GE-SX-MM850
Serial Number: FNS1234567
Connector type: LC
Wavelength: 850nm
Transfer Distance: 550m (50/125 µm OM3)
Diagnostic Monitoring: Yes
Temperature: 45.2 Celsius
Voltage: 3.30 Volts
Current: 8.5 mA
Output Power: -3.5 dBm
Receive Power: -20.1 dBm
Transmit Power High Threshold: -1.0 dBm
Transmit Power Low Threshold: -9.5 dBm
Receive Power High Threshold: 0.0 dBm
Receive Power Low Threshold: -17.0 dBmA network technician is troubleshooting a newly installed fiber link between two Cisco Catalyst 9300 switches. The link is up, but the interface shows excessive CRC errors and input errors. The technician runs 'show interfaces' and 'show interfaces transceiver details' on the suspect interface. Based on the output, what is the most likely cause of the errors?
⚠ Common exam trap
Cisco often tests the distinction between transmit power and receive power, trapping candidates who assume a low transmit power is the root cause, when in fact the receive power is the critical metric for signal integrity at the far end.
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 receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.
The output from 'show interfaces transceiver details' would show the receive power level. A receive power that is too low (e.g., below the receiver sensitivity threshold) indicates excessive signal loss, often due to dirty or damaged fiber connectors, poor splices, or excessive cable attenuation. This causes bit errors that manifest as CRC and input errors, even though the link is physically up. Option D correctly identifies this as the most likely cause.
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 for full duplex but the switchport is set to auto-negotiation, causing a duplex mismatch.
Why it's wrong here
A duplex mismatch would occur if one side were set to auto-negotiation and the other forced to 1000/full, but the output here shows 'Full-duplex' and 'auto-negotiation: true' with 1000 Mbps, so the link negotiated successfully. Duplex-mismatch errors characteristically generate late collisions, runts, and an increasing CRC count at the interface, none of which are indicated in the provided output. The CRC errors in this case are attributable to the low optical receive power, not to Layer 1 duplex framing issues.
- ✗
The SFP transceiver is faulty because the transmit power is too low at -3.5 dBm.
Why it's wrong here
Although -3.5 dBm may seem low, it falls within the normal transmit power range for an SX SFP (approximately -1 to -9.5 dBm), so the transceiver is not necessarily faulty. A failed SFP would typically report no light or an out-of-range TX value, and it would not explain why the RX power is low while TX is healthy. The symptom points to a receive-side signal loss caused by the fiber plant, not a defective SFP.
- ✗
The fiber cable is too long for the SX SFP, exceeding the distance limit.
Why it's wrong here
The SX SFP is rated for up to 550 m over OM3 multimode fiber, and the command output gives no cable-length indication that would suggest the distance limit is exceeded. If the cable were too long, the link would likely be completely down or show a much more severely degraded RX power, rather than a marginal -20.1 dBm reading. Over-distance is a possible attenuation cause, but with no measured length, connector contamination or damage is a more immediate and common explanation. Therefore, distance alone should not be selected as the root cause.
- ✓
The receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.
Why this is correct
The receive power of -20.1 dBm falls below the low alarm threshold of -17.0 dBm, while the transmit power is normal, indicating a problem in the optical path rather than the transceiver. Weak RX power causes CRC errors because the receiver has difficulty distinguishing 1s and 0s from the noise floor. Dirty or damaged fiber connectors, excessive splices, or high attenuation in the cable are the most likely culprits. Cleaning endfaces and checking patch panels should be the first troubleshooting steps.
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 receive power is too low, likely due to a dirty or damaged fiber connector or excessive attenuation.Correct answer▾
Why this is correct
The receive power of -20.1 dBm falls below the low alarm threshold of -17.0 dBm, while the transmit power is normal, indicating a problem in the optical path rather than the transceiver. Weak RX power causes CRC errors because the receiver has difficulty distinguishing 1s and 0s from the noise floor. Dirty or damaged fiber connectors, excessive splices, or high attenuation in the cable are the most likely culprits. Cleaning endfaces and checking patch panels should be the first troubleshooting steps.
✗The interface is configured for full duplex but the switchport is set to auto-negotiation, causing a duplex mismatch.Wrong answer — click to see why▾
Why this is wrong here
The interface shows Full Duplex and 1000 Mbps with no collisions or late collisions, which indicates no duplex mismatch. Duplex mismatch typically causes collisions and late collisions, which are absent here.
Why candidates choose this
Students often associate CRC errors with duplex mismatch, but CRC errors are more commonly caused by physical layer issues like signal degradation or faulty hardware, not duplex mismatch.
✗The SFP transceiver is faulty because the transmit power is too low at -3.5 dBm.Wrong answer — click to see why▾
Why this is wrong here
The transmit power of -3.5 dBm is within the normal range for SX SFP (-1.0 to -9.5 dBm), so the SFP is transmitting correctly. Low transmit power would not cause CRC errors if it is within specifications.
Why candidates choose this
Students may think that any low power reading indicates a faulty SFP, but the key is to compare against the threshold. Transmit power is often less critical than receive power for link errors.
✗The fiber cable is too long for the SX SFP, exceeding the distance limit.Wrong answer — click to see why▾
Why this is wrong here
The SX SFP supports up to 550m on OM3 fiber, and the link is operational, so distance is not the issue. Excessive CRC errors due to distance would typically cause the link to be down or unstable.
Why candidates choose this
Students may assume that fiber distance limits are always the cause of errors, but the link being up and the specific error pattern (CRC) points more to signal attenuation or connector 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
Network Topology Types
Key term
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
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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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.