CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
SwitchA# show interfaces Gi1/0/1
GigabitEthernet1/0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aabb.ccdd.0001 (bia aabb.ccdd.0001)
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 10Gbase-SR
input flow-control is off, output flow-control is off
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: 0/2000/0/0 (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
123456 packets input, 12345678 bytes
Received 1200 broadcasts (0 multicast)
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
123456 packets output, 12345678 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 Gi1/0/1 transceiver details
Diagnostic Monitoring: Yes
Temperature: 45.2 C
Voltage: 3.29 V
Current: 12.5 mA
Output Power: -2.3 dBm
Receive Power: -15.1 dBm
Transmit Fault: No
LOS: No
Nominal bit rate: 10300 Mbps
Connector type: LC
Wavelength: 850 nm
Distance: 300 m
Vendor: CISCO
Part Number: SFP-10G-SR
Serial Number: FNS1234567A network engineer is troubleshooting a link between two Cisco Catalyst 9300 switches that are connected via a 10GBASE-SR SFP+ module on each end over OM3 multimode fiber. The link is up, but the interface counters show a high number of CRC errors and runts. The engineer runs 'show interfaces Gi1/0/1' and 'show interfaces Gi1/0/1 transceiver details'. What is the most likely cause of the errors?
⚠ Common exam trap
Cisco often tests the misconception that CRC errors always indicate a faulty transceiver or cable length issue, when in reality low optical power from dirty or damaged connectors is a more common and subtle cause on fiber links.
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 optical power is too low, indicating a fiber or connector issue.
The high CRC errors and runts on a 10GBASE-SR link over OM3 fiber, combined with the 'show interfaces transceiver details' command, point to a physical-layer issue. Low receive optical power (below the receiver sensitivity threshold) causes bit errors that manifest as CRC errors and runts, even though the link is up. This is the most common cause when the fiber and transceivers are otherwise compatible.
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 SFP+ module is faulty and needs replacement.
Why it's wrong here
The SFP+ module is not faulty because digital optical monitoring (DOM) shows transmit power, temperature, voltage, and bias current all within normal operating ranges, with no transmit fault or loss-of-signal alarms. Low receive power alone does not indicate a module failure—it points to an issue in the optical path upstream of the receiver, such as contaminated fiber connectors, a damaged patch cable, or a poor splice. If the SFP+ itself were failing, you would typically see abnormal transmit power, high bias current, or LOS alarms, not simply a low RX reading with CRC errors on the interface.
- ✗
The fiber patch cables are too long, exceeding the 300-meter distance limit for 10GBASE-SR over OM3 fiber.
Why it's wrong here
The exhibit shows the distance as 300 m, which is the maximum for 10GBASE-SR over OM3 fiber. While this is at the limit, the low receive power (-15.1 dBm) suggests excessive loss beyond the expected 2.6 dB over 300 m, indicating a problem with the fiber or connectors.
- ✗
The interface speed is mismatched; the switch interface shows 1000Mb/s but the SFP+ is 10GBASE-SR.
Why it's wrong here
Although the interface shows 1000Mb/s, this is because the switch is using a GigabitEthernet interface (Gi1/0/1) that is connected via a 10GBASE-SR SFP+. The SFP+ supports 10 Gbps, but the interface is a GigabitEthernet port, so the actual speed is 1 Gbps, not 10 Gbps. However, this mismatch does not cause CRC errors; the link is up and running at 1 Gbps.
- ✓
The receive optical power is too low, indicating a fiber or connector issue.
Why this is correct
The receive power of -15.1 dBm is below the typical receive sensitivity for 10GBASE-SR (about -12.6 dBm). This causes bit errors that appear as CRC errors and runts. The transmit power is normal, so the issue is on the receive side, likely dirty connectors or a damaged fiber.
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 optical power is too low, indicating a fiber or connector issue.Correct answer▾
Why this is correct
The receive power of -15.1 dBm is below the typical receive sensitivity for 10GBASE-SR (about -12.6 dBm). This causes bit errors that appear as CRC errors and runts. The transmit power is normal, so the issue is on the receive side, likely dirty connectors or a damaged fiber.
✗The SFP+ module is faulty and needs replacement.Wrong answer — click to see why▾
Why this is wrong here
The transceiver diagnostics show no fault flags, and the module is reporting nominal bit rate and other values within range.
✗The fiber patch cables are too long, exceeding the 300-meter distance limit for 10GBASE-SR over OM3 fiber.Wrong answer — click to see why▾
Why this is wrong here
The distance itself is within spec; the issue is the receive power being too low, not the distance exceeding the limit.
✗The interface speed is mismatched; the switch interface shows 1000Mb/s but the SFP+ is 10GBASE-SR.Wrong answer — click to see why▾
Why this is wrong here
This is a configuration mismatch, but it does not directly cause CRC errors. The CRC errors are due to low receive power.
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
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.
Key term
Bit
A bit is the smallest unit of data in computing and digital communications, representing a single binary value of either 0 or 1.
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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.