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: Link 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, 1000Mb/s, media type is SFP
output flow-control is unsupported, input flow-control is unsupported
Auto-negotiation is turned on
Input queue: 0/2000/0/0 (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
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 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
SwitchA# show interfaces gigabitethernet1/0/1 transceiver
Diagnostic Monitoring Information (DOM) for SFP+ 10GBASE-LR
Temperature: 45 degrees C
Voltage: 3.3 V
Current: 15 mA
Output Power: 0.5 mW (-3 dBm)
Input Power: 0.01 mW (-20 dBm)
Laser bias current: 10 mA
SwitchB# show interfaces gigabitethernet1/0/1
GigabitEthernet1/0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is dddd.eeee.ffff (bia dddd.eeee.ffff)
Description: Link to SwitchA
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 SFP
output flow-control is unsupported, input flow-control is unsupported
Auto-negotiation is turned on
Input queue: 0/2000/0/0 (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
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 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
SwitchB# show interfaces gigabitethernet1/0/1 transceiver
Diagnostic Monitoring Information (DOM) for SFP+ 10GBASE-LR
Temperature: 42 degrees C
Voltage: 3.3 V
Current: 12 mA
Output Power: 0.4 mW (-4 dBm)
Input Power: 0.02 mW (-17 dBm)
Laser bias current: 8 mAA network engineer is troubleshooting a link between two Cisco Catalyst 9300 switches that are connected via a single-mode fiber optic cable. The link is up, but the interface counters show a high number of CRC errors and frame check sequence (FCS) errors. The interface is configured for 1000 Mbps and full duplex on both ends. What is the most likely cause of these errors?
⚠ Common exam trap
Cisco often tests the distinction between CRC errors (physical-layer signal integrity) and duplex mismatch symptoms (late collisions, runts), leading candidates to incorrectly choose a speed/duplex mismatch when the interface counters show CRC 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
✓
Faulty or dirty fiber optic cable or connectors
CRC and FCS errors indicate corruption at the data-link layer, typically caused by physical-layer issues such as signal degradation. Since the link is up and both ends are configured for 1000 Mbps full duplex, a speed/duplex mismatch is ruled out. Dirty or faulty single-mode fiber connectors or the cable itself can introduce optical signal loss or reflections, leading to bit errors that manifest as CRC/FCS errors.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A speed or duplex mismatch between the two switches
Why it's wrong here
A speed or duplex mismatch would manifest as negotiation failures, late collisions, or CRC errors, but both interfaces are operating at 1000 Mb/s full-duplex with auto-negotiation enabled. On fiber connections at GbE speeds, half-duplex is rarely supported, and the DOM (Digital Optical Monitoring) outputs do not indicate any physical-layer negotiation problem. The optical receive power, not the electrical link parameters, is the anomaly here.
- ✓
Faulty or dirty fiber optic cable or connectors
Why this is correct
The diagnostic optical monitoring (DOM) reports a receive power of -20 dBm on SwitchA, which falls well below the typical receive sensitivity range for 10GBASE-LR modules (often around -14.4 dBm for 10 Gbps, though less sensitive for 1 Gbps). Such excessive optical loss is usually caused by contamination on the fiber end faces, damaged fiber, or poor splices/connectors. Since the transmit power on the remote side may be within normal range, the large difference points to a physical-layer attenuation rather than a module failure.
- ✗
Incorrect SFP+ module type inserted in the switch
Why it's wrong here
The DOM output identifies both units as SFP+ 10GBASE-LR modules, which are designed for single-mode fiber and use 1310 nm light. This is perfectly compatible with 1000 Mbps operation on single-mode fiber because SFP+ modules can often downgrade their line rate, and the required fiber and wavelength are identical to those used for 1 GbE LR links. If an SFP or incorrect 10GBASE-SR model were inserted, the DOM would show a different part number and likely the link would not come up at all, so the module type is not the cause of low receive power.
- ✗
Excessive cable length beyond the distance limit
Why it's wrong here
Exceeding the maximum distance (10 km for 10GBASE-LR) could indeed cause optical attenuation, but the DOM output gives no indication of cable length, and typical data-center patching is far shorter than this limit. The steep -20 dBm reading is more consistent with a point failure such as dirt or a break rather than distributed attenuation over a long span. Fiber attenuation is about 0.3-0.4 dB/km at 1310 nm, so reaching -20 dBm would require absurdly long cabling within a switch-to-switch link.
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.
✓Faulty or dirty fiber optic cable or connectorsCorrect answer▾
Why this is correct
The diagnostic optical monitoring (DOM) reports a receive power of -20 dBm on SwitchA, which falls well below the typical receive sensitivity range for 10GBASE-LR modules (often around -14.4 dBm for 10 Gbps, though less sensitive for 1 Gbps). Such excessive optical loss is usually caused by contamination on the fiber end faces, damaged fiber, or poor splices/connectors. Since the transmit power on the remote side may be within normal range, the large difference points to a physical-layer attenuation rather than a module failure.
✗A speed or duplex mismatch between the two switchesWrong answer — click to see why▾
Why this is wrong here
The exhibit confirms consistent speed and duplex settings.
✗Incorrect SFP+ module type inserted in the switchWrong answer — click to see why▾
Why this is wrong here
The modules are correctly identified and match the fiber type.
✗Excessive cable length beyond the distance limitWrong answer — click to see why▾
Why this is wrong here
There is no evidence of the cable length in the exhibit, and the low power is more indicative of a physical fault rather than simply exceeding distance limits.
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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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.