CCNA Network Infrastructure and Connectivity Practice Question
Network Topology
You are connected to R1 via the console. R1 and R2 are connected via a fiber link using SFPs. The link is not coming up. Configure the correct SFP type on R1's interface GigabitEthernet0/0 to support the required 2 km distance, and fix any auto-negotiation or speed/duplex misconfiguration so that the link becomes operational.
⚠ Common exam trap
Do not assume auto-negotiation is always required; fiber SFPs use fixed parameters. Also, remember that 1000BASE-SX is for short distances (up to 550 m), while 1000BASE-LX supports longer distances (up to 10 km).
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 SFP with 1000BASE-LX, configure 'no negotiation auto' on GigabitEthernet0/0, and remove 'speed 1000' and 'duplex full'.
For a 2 km fiber link, a 1000BASE-LX SFP is required (supports up to 10 km). Fiber SFPs like 1000BASE-LX typically do not support auto-negotiation; speed and duplex are fixed. Therefore, the interface should have 'no negotiation auto' configured and no explicit speed/duplex commands. Option A implements these changes. Option B uses 1000BASE-SX, which is suitable only for up to 550 m. Option C leaves auto-negotiation enabled, which is unsupported. Option D sets speed to 100, which is incompatible with a GigabitEthernet interface.
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 SFP with 1000BASE-LX, configure 'no negotiation auto' on GigabitEthernet0/0, and remove 'speed 1000' and 'duplex full'.
Why this is correct
Correct. 1000BASE-LX supports up to 10 km, sufficient for 2 km. Fiber SFPs don't support auto-negotiation, so 'no negotiation auto' is required. The speed and duplex are fixed by the SFP, so removing the explicit 'speed 1000' and 'duplex full' is appropriate.
- ✗
Replace SFP with 1000BASE-SX, configure 'negotiation auto' on GigabitEthernet0/0, and keep 'speed 1000' and 'duplex full'.
Why it's wrong here
1000BASE-SX is a multimode-fiber optic only, with a maximum reach of 550 meters, which is insufficient for the 2 km link. Moreover, Gigabit Ethernet over fiber does not use the IEEE 802.3 auto-negotiation mechanism—'negotiation auto' would be invalid on an SFP-based interface. Explicitly setting 'speed 1000' and 'duplex full' is also redundant at best and can cause the interface to revert to a non-standard configuration, so this option fails both on distance and on interface settings.
- ✗
Replace SFP with 1000BASE-LX, configure 'negotiation auto' on GigabitEthernet0/0, and keep 'speed 1000' and 'duplex full'.
Why it's wrong here
While 1000BASE-LX does provide the required 2 km reach, the command 'negotiation auto' is not applicable to fiber SFP links. 1000BASE-X (fiber) relies on a fixed 1000 Mbps full-duplex signaling negotiated through a different link synchronization process, not the copper auto-negotiation that negotiates speed and duplex. Issuing 'negotiation auto' on a fiber port can prevent the optical link from coming up, and leaving 'speed 1000' and 'duplex full' is unnecessary because the SFP already determines those parameters; the correct fix is to remove them.
- ✗
Replace SFP with 1000BASE-LX, configure 'no negotiation auto' on GigabitEthernet0/0, and configure 'speed 100' and 'duplex full'.
Why it's wrong here
The 1000BASE-LX SFP is specifically rated for 1000 Mbps and cannot operate at 100 Mbps, so configuring 'speed 100' creates an immediate mismatch between the interface and the SFP's fixed gigabit rate. Even though 'no negotiation auto' correctly disables auto-negotiation, the speed command forces the MAC/PHY to use a speed that the fiber PMD does not support, and the link will not come up. On Cisco routers, SFP-based GigabitEthernet interfaces normally have no 'speed' or 'duplex' commands in the running configuration; the correct action is to delete them so the SFP defaults to 1000/full.
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 SFP with 1000BASE-LX, configure 'no negotiation auto' on GigabitEthernet0/0, and remove 'speed 1000' and 'duplex full'.Correct answer▾
Why this is correct
Correct. 1000BASE-LX supports up to 10 km, sufficient for 2 km. Fiber SFPs don't support auto-negotiation, so 'no negotiation auto' is required. The speed and duplex are fixed by the SFP, so removing the explicit 'speed 1000' and 'duplex full' is appropriate.
✗Replace SFP with 1000BASE-SX, configure 'negotiation auto' on GigabitEthernet0/0, and keep 'speed 1000' and 'duplex full'.Wrong answer — click to see why▾
Why this is wrong here
The SFP type is incorrect for the distance; 1000BASE-SX cannot reach 2 km.
Why candidates choose this
Candidates may think SX is standard for short distances and assume auto-negotiation is always required.
✗Replace SFP with 1000BASE-LX, configure 'negotiation auto' on GigabitEthernet0/0, and keep 'speed 1000' and 'duplex full'.Wrong answer — click to see why▾
Why this is wrong here
Auto-negotiation is not supported on fiber interfaces; it should be disabled with 'no negotiation auto'.
Why candidates choose this
Candidates may believe auto-negotiation is needed for all Ethernet links, not realizing fiber SFPs have fixed parameters.
✗Replace SFP with 1000BASE-LX, configure 'no negotiation auto' on GigabitEthernet0/0, and configure 'speed 100' and 'duplex full'.Wrong answer — click to see why▾
Why this is wrong here
Setting 'speed 100' on a 1000BASE-LX SFP is invalid because the SFP only operates at 1000 Mbps; the link will not come up.
Why candidates choose this
Candidates may think keeping speed/duplex is harmless, but the question expects a clean configuration that removes misconfigurations.
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
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
Key term
Auto-negotiation
Auto-negotiation is an Ethernet feature that allows two connected devices to automatically agree on the best possible speed and duplex mode for their link.
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