220-1101 Networking Practice Question
A network technician connects two switches together using a straight-through Cat6 Ethernet cable. Both switches are managed and are known to be working. After connecting, the link LED on Switch A is off, and the link LED on Switch B is also off. The technician has verified that both switch ports are enabled and not administratively shut down. Which of the following is the MOST likely cause of the problem?
⚠ Common exam trap
Many exam-takers assume a straight-through cable is always correct for switch-to-switch connections, but the CompTIA 220-1101 exam tests the nuance that auto-MDIX support determines whether a straight-through or crossover cable is needed, and a lack of auto-MDIX on at least one switch will cause the link to fail.
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
✓
At least one switch does not support auto-MDIX.
When two switches are connected directly, auto-MDIX (Automatic Medium-Dependent Interface Crossover) allows either a straight-through or crossover cable to be used by automatically negotiating the transmit/receive pairs. If at least one switch does not support auto-MDIX, a straight-through cable will not work because the transmit pins on one switch will connect to transmit pins on the other, causing a link failure. Since both link LEDs are off and the ports are enabled, the most likely cause is that one or both switches lack auto-MDIX support, requiring a crossover cable instead.
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 a crossover cable.
Why it's wrong here
The problem explicitly states the cable is straight-through, so a crossover cable is not the cause; in fact, using a crossover between two like devices would be the correct solution if auto-MDIX is unavailable. A crossover cable physically swaps transmit and receive pairs, which is exactly what needs to happen for switch-to-switch links. Since the cable here is straight-through, this option contradicts the scenario.
- ✗
The cable length exceeds 100 meters.
Why it's wrong here
A cable run that exceeds 100 meters will likely suffer from attenuation and signal degradation, causing link failure, but nothing in the question suggests the cable is that long. Even if the run were shorter than 100 meters, a good straight-through cable would still produce a link light provided the switch ports support auto-MDIX or the cable is wired correctly. The failure to link here stems from a transmit/receive pair mismatch, not distance.
- ✗
One of the switches has the wrong VLAN assignment.
Why it's wrong here
VLAN configuration is a Layer 2 function; it determines which broadcast domains traffic is forwarded to after the physical link is established. A misassigned VLAN, default VLAN mismatch, or trunk/access port error would not prevent the port LEDs from lighting up — the physical layer would still come up. Since the symptom is no link, this option incorrectly conflates data-plane isolation with physical connectivity.
- ✓
At least one switch does not support auto-MDIX.
Why this is correct
This is correct: without auto-MDIX on at least one switch, the transmit pins (1,2) of one port will connect to the transmit pins of the other, preventing link. Auto-MDIX allows a port to automatically swap its transmit and receive pairs so a straight-through cable works between similar devices. If one switch lacks this feature, the pair mismatch remains and the link stays down unless a crossover cable is used.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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