CCNA Network Infrastructure and Connectivity Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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
The interface statistics show both switches are already configured for full duplex and 1000 Mbps, and optical SFP modules do not expose manual speed/duplex tuning like copper interfaces do. Even if the settings could be forced, a down/down state is caused by a Layer 1 physical mismatch, not by negotiation parameters. Matching the existing operating parameters will not resolve the SFP media-type incompatibility.
- ✓
Replace the 1000BaseSX SFP on SW2 with a 1000BaseLX SFP.
Why this is correct
SW1's 1000BaseLX SFP operates at 1300 nm on single-mode fiber, while SW2's 1000BaseSX SFP operates at 850 nm on multimode fiber; the wavelength difference means the electrical-to-optical conversion at the receiver cannot interpret the incoming light signal. With a down/down status, the root cause is a media-type mismatch, so replacing the SX module with an LX SFP on SW2 ensures both ends use the same 1300 nm wavelength and fiber mode. This allows the physical layer to establish a signal and bring the interface up.
- ✗
Enable MDIX on both interfaces to allow automatic crossover detection.
Why it's wrong here
MDIX (Medium Dependent Interface Crossover) is a copper Ethernet feature that automatically corrects for straight-through versus crossover cabling by swapping transmit and receive pairs. Fiber optic links use separate strands for transmit and receive, so they have no concept of a cable crossover, and MDIX is not implemented in optical SFP modules. Enabling this feature would have no effect on the fiber link's ability to establish a Layer 1 signal.
- ✗
Change the VLAN assignment on SW2's interface to match that of SW1.
Why it's wrong here
A VLAN mismatch between the two ports would cause the interface to remain up/up at both Layer 1 and Layer 2, but traffic would not be forwarded because the access or trunk VLAN identifiers do not agree. The reported down/down status indicates a failure at the physical layer, meaning the optical modules are not producing or receiving light properly. Since the problem exists before any 802.1Q or access VLAN rules are applied, changing the VLAN assignment cannot fix the link state.
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
SW1's 1000BaseLX SFP operates at 1300 nm on single-mode fiber, while SW2's 1000BaseSX SFP operates at 850 nm on multimode fiber; the wavelength difference means the electrical-to-optical conversion at the receiver cannot interpret the incoming light signal. With a down/down status, the root cause is a media-type mismatch, so replacing the SX module with an LX SFP on SW2 ensures both ends use the same 1300 nm wavelength and fiber mode. This allows the physical layer to establish a signal and bring the interface up.
✗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?”
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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.