The correct answer is to replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP. This resolves the SFP wavelength mismatch because single-mode fiber requires long-wavelength LX optics (around 1310 nm), while SX optics use short-wavelength light (850 nm) designed for multimode fiber. When an SX module is plugged into single-mode fiber, the signal attenuates and scatters too quickly for the remote receiver to detect, causing the down/down state on SW2 even though SW1 sees its own transmit as up/up. On the CCNA 200-301 v2 exam, this scenario tests your understanding of fiber optic transceiver compatibility and the physical layer troubleshooting process—a common trap is assuming both ends must match in brand or speed, when the real issue is wavelength and fiber type. Remember the memory tip: “LX for long haul, SX for short”—if the cable is single-mode, always choose LX.
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
SW1# show interfaces gigabitethernet0/1
GigabitEthernet0/1 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, 1000Mbps, link type is auto, media type is 1000BaseLX
output flow-control is off, input flow-control is off
...
SW2# show interfaces gigabitethernet0/1
GigabitEthernet0/1 is down, line protocol is down
Hardware is Gigabit Ethernet, address is dddd.eeee.ffff (bia dddd.eeee.ffff)
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 1000BaseSX
output flow-control is off, input flow-control is off
...
A network technician is troubleshooting a link between two Cisco switches, SW1 and SW2, connected via a single-mode fiber optic cable. The interface on SW1 is up/up, but the interface on SW2 remains down/down. The technician has verified that the fiber cable is not damaged and that the SFP modules are correctly seated. Which configuration change should the technician make to resolve the issue?
SW1# show interfaces gigabitethernet0/1
GigabitEthernet0/1 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, 1000Mbps, link type is auto, media type is 1000BaseLX
output flow-control is off, input flow-control is off
...
SW2# show interfaces gigabitethernet0/1
GigabitEthernet0/1 is down, line protocol is down
Hardware is Gigabit Ethernet, address is dddd.eeee.ffff (bia dddd.eeee.ffff)
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 1000BaseSX
output flow-control is off, input flow-control is off
...
A
Configure the interface on SW2 to use the same speed and duplex settings as SW1.
Why wrong: Both interfaces are already set to Full Duplex and 1000Mbps, and fiber links typically auto-negotiate correctly. This is not the root cause.
B
Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP.
The SFP types do not match—SW1 is LX (single-mode) and SW2 is SX (multimode). Using a consistent LX SFP on both ends ensures proper communication over single-mode fiber.
C
Enable MDIX on both interfaces to allow automatic crossover detection.
Why wrong: MDIX is used for copper Ethernet cables to correct for straight-through vs. crossover issues. Fiber optic connections do not use MDIX.
D
Change the VLAN assignment on SW2's interface to match that of SW1.
Why wrong: VLAN mismatch would cause the interface to be up/up but not forward traffic, not the down/down state seen here. The issue is at Layer 1.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP.
The issue is that SW1 is up/up but SW2 is down/down, indicating a unidirectional link. Since the fiber cable and SFP seating are verified, the most likely cause is an SFP wavelength mismatch. SW1 likely has a 1000BaseLX SFP (long-wavelength, single-mode), while SW2 has a 1000BaseSX SFP (short-wavelength, multimode). Single-mode fiber requires LX optics; SX optics are designed for multimode fiber and will not produce a signal that can be received correctly over single-mode fiber, causing the remote interface to remain down. Replacing the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP resolves the wavelength incompatibility.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Configure the interface on SW2 to use the same speed and duplex settings as SW1.
Why it's wrong here
Both interfaces are already set to Full Duplex and 1000Mbps, and fiber links typically auto-negotiate correctly. This is not the root cause.
✓
Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP.
Why this is correct
The SFP types do not match—SW1 is LX (single-mode) and SW2 is SX (multimode). Using a consistent LX SFP on both ends ensures proper communication over single-mode fiber.
Related concept
Read the scenario before looking for a memorised answer.
✗
Enable MDIX on both interfaces to allow automatic crossover detection.
Why it's wrong here
MDIX is used for copper Ethernet cables to correct for straight-through vs. crossover issues. Fiber optic connections do not use MDIX.
✗
Change the VLAN assignment on SW2's interface to match that of SW1.
Why it's wrong here
VLAN mismatch would cause the interface to be up/up but not forward traffic, not the down/down state seen here. The issue is at Layer 1.
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.
✓Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP.Correct answer▾
Why this is correct
The SFP types do not match—SW1 is LX (single-mode) and SW2 is SX (multimode). Using a consistent LX SFP on both ends ensures proper communication over single-mode fiber.
✗Configure the interface on SW2 to use the same speed and duplex settings as SW1.Wrong answer — click to see why▾
Why this is wrong here
On fiber optic links, speed and duplex are typically fixed (e.g., 1000 Mbps full duplex) and do not require manual configuration; auto-negotiation is standard for Gigabit Ethernet over fiber. Since the interface on SW1 is up/up, the settings are already compatible, so this change would not resolve the down/down state on SW2.
Why candidates choose this
Students often associate speed/duplex mismatches with interface issues, but this is more common on copper Ethernet links. They may overlook that fiber links have different physical layer characteristics.
✗Enable MDIX on both interfaces to allow automatic crossover detection.Wrong answer — click to see why▾
Why this is wrong here
MDIX (Medium Dependent Interface Crossover) is a feature for copper Ethernet cables to automatically correct for straight-through vs. crossover cable issues. Fiber optic connections do not use MDIX because they use separate transmit and receive fibers, so enabling MDIX has no effect on fiber links.
Why candidates choose this
Test-takers may confuse MDIX with general auto-negotiation or crossover correction, not realizing it applies only to copper media.
✗Change the VLAN assignment on SW2's interface to match that of SW1.Wrong answer — click to see why▾
Why this is wrong here
A VLAN mismatch would cause the interface to be up/up but not forward traffic (Layer 2 issue), not the down/down state observed. The down/down state indicates a Layer 1 problem, such as a physical or optical incompatibility.
Why candidates choose this
Students often think VLAN mismatches cause interface down states, but they actually affect traffic flow after the link is established. The symptom of down/down points to a physical layer fault.
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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that fiber link issues are always due to physical damage or seating, when in fact the most common exam trap is an SFP type mismatch (SX vs. LX) on single-mode fiber, causing a unidirectional link.
Detailed technical explanation
How to think about this question
1000BaseSX (IEEE 802.3z) uses 850 nm wavelength and is designed for multimode fiber with a maximum distance of 550 meters, while 1000BaseLX uses 1310 nm wavelength and can run on both single-mode and multimode fiber (with mode-conditioning patch cables for multimode). On single-mode fiber, the 850 nm signal from an SX SFP attenuates rapidly and does not couple efficiently, resulting in a loss of light at the receiver, which keeps the remote interface in down/down state. The 'up/up' on SW1 indicates it is receiving light from SW2's SX SFP (though possibly weak), but SW2's receiver cannot detect SW1's LX signal due to the wavelength mismatch.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP. — The issue is that SW1 is up/up but SW2 is down/down, indicating a unidirectional link. Since the fiber cable and SFP seating are verified, the most likely cause is an SFP wavelength mismatch. SW1 likely has a 1000BaseLX SFP (long-wavelength, single-mode), while SW2 has a 1000BaseSX SFP (short-wavelength, multimode). Single-mode fiber requires LX optics; SX optics are designed for multimode fiber and will not produce a signal that can be received correctly over single-mode fiber, causing the remote interface to remain down. Replacing the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP resolves the wavelength incompatibility.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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