220-1101 Networking Practice Question
A technician installs a new VoIP phone that receives power via PoE from a managed switch. The phone powers on but cannot register with the call server. A PC connected to the phone's passthrough port obtains an IP address and accesses the internet normally. Which of the following is the most likely cause of the phone's failure?
⚠ Common exam trap
Test-takers frequently assume a working PC passthrough means the entire link is fine, overlooking that voice and data VLANs are separate logical networks, so a voice VLAN mismatch can break phone registration while data connectivity remains unaffected.
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
✓
The switch port has a VLAN mismatch for the voice VLAN
The phone powers on via PoE and the PC behind it works, proving Layer 1 and basic Layer 2 connectivity are fine. The phone cannot register with the call server, which typically requires it to be on the correct voice VLAN. A VLAN mismatch for the voice VLAN means the switch port is configured to tag voice traffic with a different VLAN ID than the phone expects, preventing the phone from communicating with the call server.
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 switch port is configured with a voice VLAN that the phone is not using
Why it's wrong here
This option reverses cause and effect: if the voice VLAN were correctly configured on the switch port, the phone would automatically use it when transmitting, regardless of its initial settings. The real issue is that the phone is not receiving the voice VLAN assignment—either the port lacks the voice VLAN or the configuration is mismatched. Simply saying the phone 'is not using' the voice VLAN describes the symptom, not a separate root cause, so this is incorrect.
When this WOULD be correct
A technician installs a VoIP phone that receives power via PoE but cannot register. The switch port is configured with a voice VLAN, but the phone's firmware is set to use the default data VLAN instead. The PC works because it's on the data VLAN.
- ✓
The switch port has a VLAN mismatch for the voice VLAN
Why this is correct
The switch port is likely configured for data VLAN only, while the VoIP phone expects to tag its traffic with a separate voice VLAN ID. This mismatch prevents the phone from communicating with the call controller or obtaining an IP address on the correct subnet, even though PoE powers the device. The PC behind the phone works because it uses the untagged data VLAN, which is correctly configured, while the phone's voice traffic is misrouted.
- ✗
The Ethernet cable between the switch and the phone is faulty
Why it's wrong here
A faulty Ethernet cable would typically cause link negotiation to fail or produce intermittent connectivity, but here the phone powers on via PoE and the PC passes traffic through the phone's passthrough port. Since PoE delivers power over the same wire pairs used for data, successful power delivery indicates the pairs are functional enough to carry link and data frames. Thus, a cable fault is highly unlikely because the physical link is established and working for both power and data.
When this WOULD be correct
A technician installs a VoIP phone that fails to power on and a PC connected to the phone's passthrough port also has no link. The switch port shows no errors. In this scenario, a faulty cable would be the likely cause.
- ✗
The phone's MAC address is filtered on the network
Why it's wrong here
MAC address filtering on the switch port would drop frames from any unauthorized MAC, affecting both the phone and the PC behind it. Since the PC communicates normally through the passthrough port, the port is not applying MAC-based security restrictions. Additionally, MAC filtering is typically configured per port, and if it were active, the phone's frames would be ignored just like any other unknown MAC, but the PC's success proves no such filter is in place.
When this WOULD be correct
A technician installs a new VoIP phone that cannot connect to the network at all. A PC connected to the phone's passthrough port also fails to obtain an IP address. The switch logs show the phone's MAC address is blocked. In this scenario, MAC filtering would be the likely cause.
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 220-1101 exam frequently reuses these exact scenarios with slightly different constraints.
✓The switch port has a VLAN mismatch for the voice VLANCorrect answer▾
Why this is correct
The switch port is likely configured for data VLAN only, while the VoIP phone expects to tag its traffic with a separate voice VLAN ID. This mismatch prevents the phone from communicating with the call controller or obtaining an IP address on the correct subnet, even though PoE powers the device. The PC behind the phone works because it uses the untagged data VLAN, which is correctly configured, while the phone's voice traffic is misrouted.
✗The switch port is configured with a voice VLAN that the phone is not usingWrong answer — click to see why▾
Why this is wrong here
The phone powers on and the PC works, so the switch port is providing power and data. The issue is registration, which points to a VLAN mismatch (the phone is on the wrong VLAN for the call server), not the phone refusing to use a configured voice VLAN.
★ When this WOULD be the correct answer
A technician installs a VoIP phone that receives power via PoE but cannot register. The switch port is configured with a voice VLAN, but the phone's firmware is set to use the default data VLAN instead. The PC works because it's on the data VLAN.
Why candidates choose this
Candidates may confuse 'not using a configured voice VLAN' with a VLAN mismatch, thinking the phone's failure to register is due to it ignoring the voice VLAN configuration, rather than the switch port being misconfigured for the voice VLAN.
✗The Ethernet cable between the switch and the phone is faultyWrong answer — click to see why▾
Why this is wrong here
A faulty Ethernet cable would likely cause the phone to not power on or the PC to not obtain an IP address, but the PC works fine, indicating the cable is functional.
★ When this WOULD be the correct answer
A technician installs a VoIP phone that fails to power on and a PC connected to the phone's passthrough port also has no link. The switch port shows no errors. In this scenario, a faulty cable would be the likely cause.
Why candidates choose this
Candidates may assume any connectivity issue is due to a bad cable, overlooking that the PC's successful operation rules out a cable fault.
✗The phone's MAC address is filtered on the networkWrong answer — click to see why▾
Why this is wrong here
The PC connected to the phone's passthrough port obtains an IP address and accesses the internet normally, indicating that the network connection and MAC filtering are not blocking traffic. The phone's failure to register is specifically related to voice VLAN configuration, not MAC filtering.
★ When this WOULD be the correct answer
A technician installs a new VoIP phone that cannot connect to the network at all. A PC connected to the phone's passthrough port also fails to obtain an IP address. The switch logs show the phone's MAC address is blocked. In this scenario, MAC filtering would be the likely cause.
Why candidates choose this
Candidates may think that MAC filtering is a common security measure that could block the phone, but they overlook that the PC behind the phone works fine, which rules out MAC filtering at the switch port level.
Analysis generated from the official 220-1101blueprint 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
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