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220-1201 Practice Question: A network administrator is troubleshooting a…
A network administrator is troubleshooting a 1000BASE-T connection that is only running at 100 Mbps. The cable is Cat5e and the switch supports Gigabit. The link light is green but the speed is capped. What is the most likely cause?
⚠ Common exam trap
A common mistake is assuming a green link light guarantees Gigabit speed. In CompTIA exams, remember that 1000BASE-T requires all four wire pairs; if any pair fails, the link falls back to 100BASE-TX using only two pairs. The issue is physical, not configuration.
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
✓
One of the four wire pairs is broken or shorted.
1000BASE-T requires all four wire pairs (eight wires) to operate. If one pair is broken or shorted, the link will auto-negotiate down to 100BASE-TX, which only needs two pairs. The green link light indicates a physical connection, but the speed is capped because the PHY detects the missing pair and falls back to the slower standard.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The cable is too long for Gigabit.
Why it's wrong here
Cat5e supports Gigabit up to 100 meters, so length is not the issue unless it exceeds that.
- ✓
One of the four wire pairs is broken or shorted.
Why this is correct
Gigabit requires all four pairs; if one pair fails, the device auto-negotiates to 100 Mbps using two pairs.
- ✗
The network card is set to 100 Mbps manually.
Why it's wrong here
Manual setting could cause this, but it is less common than a physical cable fault.
- ✗
The switch port is configured for 100 Mbps.
Why it's wrong here
Switch configuration could limit speed, but the scenario says the switch supports Gigabit, so it's likely auto-negotiating.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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