CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
Switch# 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)
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 1000BaseLX
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: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 1000 bits/sec, 2 packets/sec
5 minute output rate 1000 bits/sec, 2 packets/sec
1234 packets input, 150000 bytes, 0 no buffer
Received 100 broadcasts (0 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
567 packets output, 100000 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 outAn engineer is deploying a new Cisco Catalyst 9300 switch in a campus wiring closet. The uplink to the distribution switch uses a 1000BASE-LX SFP module. After connecting the fiber, the interface shows 'up/up' but the engineer notices that the 'input errors' counter is incrementing rapidly, with many CRC errors, runts, and giants being reported. What is the most likely cause of these input errors?
⚠ Common exam trap
Learners often misdiagnose runts and giants as a duplex mismatch, but when CRC errors are present alongside them, the issue is a physical-layer impairment—such as excessive fiber distance—rather than a configuration error.
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
✓
Check the fiber distance and ensure it is within the 5 km limit for 1000BASE-LX; if over, use a single-mode fiber extender.
The 1000BASE-LX standard uses long-wavelength laser optics (1300 nm) over single-mode fiber with a maximum distance of 5 km. Exceeding this limit causes signal attenuation and dispersion, generating bit errors that corrupt the frame check sequence (FCS). Cisco IOS counts these as CRC errors. Runts (frames shorter than 64 bytes) and giants (frames longer than 1518 bytes) also appear because the damaged frames are misinterpreted. Option B correctly identifies the distance limit as the root cause and recommends verifying it or using a fiber extender. Options A and D propose incorrect media, and option C would break the link since 1000BASE-LX operates at fixed 1000/full speed.
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 SFP with a 1000BASE-SX module.
Why it's wrong here
1000BASE-SX uses 850nm light and is designed for multimode fiber, with a maximum reach of about 220-550m depending on fiber grade. If the installed fiber is single-mode, an SX module will not generate a usable signal because the core diameter is too small for the 850nm wavelength to propagate, causing link failure, no light, or excessive errors. Even if the fiber were multimode, SX cannot cover the same distance as LX, so this replacement would not solve the attenuation problem.
- ✓
Check the fiber distance and ensure it is within the 5 km limit for 1000BASE-LX; if over, use a single-mode fiber extender.
Why this is correct
1000BASE-LX SFP modules are rated for a maximum distance of 5km over single-mode fiber using 1310nm light. If the actual fiber distance exceeds this, the received optical signal power falls below the receiver sensitivity, producing bit errors and CRC errors on the interface. The correct action is to verify the link distance and either use a fiber extender/repeater or upgrade to a longer-reach optic such as 1000BASE-ZX (up to 70km) to regain a positive link budget.
- ✗
Configure the interface with 'speed 100' and 'duplex full' to match the SFP capabilities.
Why it's wrong here
The 1000BASE-LX SFP is a gigabit-only transceiver with a fixed line rate of 1000 Mbps; it does not support 100BASE-TX or any speed negotiation to 100 Mbps. Forcing 'speed 100' on the interface will cause a speed mismatch, and the interface will fail to come up or will continuously flap with errors. The 'duplex full' command is also irrelevant because it does not affect the optics' physical-layer characteristics such as wavelength, power, or distance.
- ✗
Replace the fiber patch cable with a CAT6a copper cable and use a 1000BASE-T SFP.
Why it's wrong here
Replacing the fiber with CAT6a and a 1000BASE-T SFP would convert the link to copper Ethernet, which is limited to 100 meters. If the current link distance needs to span more than 100m, copper is physically impossible, and this change does not address the root cause of signal attenuation over long fiber spans. Additionally, you would be introducing a completely different media type without fixing the distance issue, and the switch interface may not support the specific 1000BASE-T SFP model.
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.
✓Check the fiber distance and ensure it is within the 5 km limit for 1000BASE-LX; if over, use a single-mode fiber extender.Correct answer▾
Why this is correct
1000BASE-LX SFP modules are rated for a maximum distance of 5km over single-mode fiber using 1310nm light. If the actual fiber distance exceeds this, the received optical signal power falls below the receiver sensitivity, producing bit errors and CRC errors on the interface. The correct action is to verify the link distance and either use a fiber extender/repeater or upgrade to a longer-reach optic such as 1000BASE-ZX (up to 70km) to regain a positive link budget.
✗Replace the SFP with a 1000BASE-SX module.Wrong answer — click to see why▾
Why this is wrong here
1000BASE-SX is designed for multimode fiber and supports distances up to 550 meters. If the existing fiber is single-mode, using an SX module will not work and could cause link issues or errors. The problem described involves runts and giants, which are typically due to distance or attenuation, not the fiber type mismatch.
Why candidates choose this
Students may confuse SX and LX, thinking that SX is a common fix for fiber issues, or they may assume that any SFP will work on any fiber. However, SX is for multimode, while LX is for single-mode, and the question implies single-mode fiber is in use.
✗Configure the interface with 'speed 100' and 'duplex full' to match the SFP capabilities.Wrong answer — click to see why▾
Why this is wrong here
The 1000BASE-LX SFP operates only at 1000 Mbps and does not support 100 Mbps. Forcing speed 100 would cause the interface to go down or generate errors due to speed mismatch. The runts and giants are not caused by speed/duplex mismatch in this scenario.
Why candidates choose this
Students often associate input errors with speed/duplex mismatches, which is a common cause on copper interfaces. However, on fiber, speed is fixed, and duplex is always full, so this is not applicable.
✗Replace the fiber patch cable with a CAT6a copper cable and use a 1000BASE-T SFP.Wrong answer — click to see why▾
Why this is wrong here
Replacing fiber with copper would require different SFPs and cabling, but the issue is likely distance-related on the fiber link. Runts and giants due to distance are not resolved by changing media type; the root cause must be addressed.
Why candidates choose this
Students might think that copper is more reliable or that changing media type can fix errors, but the problem is specific to fiber distance limitations.
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
Half-Duplex vs Full-Duplex
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
Frame
A frame is a unit of data at the Data Link layer of networking, containing header, payload, and trailer information for local network delivery.
About these practice questions
One of 1,389 original 200-301 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
JA
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.