CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
SwitchA# show interfaces gigabitethernet 0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
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 off, input 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 00:05:00
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, 1 packets/sec
5 minute output rate 1000 bits/sec, 1 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 outA network engineer is troubleshooting a link between two Cisco switches that are connected via a 1000BASE-LX SFP transceiver. Hosts on VLAN 10 on Switch A cannot ping the default gateway on Switch B. The interface on Switch A is up, but the engineer notices CRC errors incrementing. What should the engineer do to resolve this issue?
⚠ Common exam trap
Cisco often tests the misconception that CRC errors always indicate a bad cable or transceiver, when in fact the root cause can be a mode mismatch between LX optics and multimode fiber that is fixed with a mode-conditioning patch cable rather than replacing the fiber or transceiver.
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
✓
Use a mode-conditioning patch cable between the SFP and the fiber patch panel
The 1000BASE-LX SFP transceiver operates at 1310 nm over single-mode fiber (SMF). When connected to multimode fiber (MMF), the larger core diameter causes excessive modal dispersion, leading to CRC errors. A mode-conditioning patch cable (mode-conditioning patch cord) launches the laser off-center into the MMF core, reducing differential mode delay and restoring proper signal integrity. This is a known Cisco best practice for LX transceivers on MMF links.
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 transceiver
Why it's wrong here
1000BASE-SX is for multimode fiber only, but the transceiver is already LX (single-mode).
- ✓
Use a mode-conditioning patch cable between the SFP and the fiber patch panel
Why this is correct
A mode-conditioning patch cable corrects the mismatch when using a single-mode transceiver over multimode fiber, preventing signal dispersion.
- ✗
Configure the same VLAN on both switch interfaces
Why it's wrong here
VLAN mismatch would cause the line protocol to be down, not CRC errors.
- ✗
Replace the fiber cable with a single-mode fiber cable
Why it's wrong here
While this would also fix the mismatch, the question asks for the most likely cause given the symptom of CRC errors.
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.
✓Use a mode-conditioning patch cable between the SFP and the fiber patch panelCorrect answer▾
Why this is correct
A mode-conditioning patch cable corrects the mismatch when using a single-mode transceiver over multimode fiber, preventing signal dispersion.
✗Replace the SFP with a 1000BASE-SX transceiverWrong answer — click to see why▾
Why this is wrong here
Using SX would not fix the issue because the fiber is multimode and LX is designed for longer distances; the problem is the cable type mismatch, not the transceiver type.
✗Configure the same VLAN on both switch interfacesWrong answer — click to see why▾
Why this is wrong here
The interface is up/up, so VLAN mismatch is not the issue.
✗Replace the fiber cable with a single-mode fiber cableWrong answer — click to see why▾
Why this is wrong here
Although replacing with single-mode fiber would work, the exhibit does not show CRC errors, so the problem is more likely a mode-conditioning issue.
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?”
Visual reference
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Related to this question
Learn chapter
ICMP and Ping
Key term
Default gateway
A default gateway is a network device, typically a router, that acts as the exit point for traffic from a local network to other networks, including the internet.
Key term
CRC
CRC (Cyclic Redundancy Check) is an error-detecting code used to detect accidental changes to raw data in digital networks.
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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.