PoE Budget Exceeded: VoIP Phone Fails to Power On
A technician is configuring a small office network with a PoE-managed switch and several VoIP phones. All phones are connected and working except one. On the non-working phone, the Ethernet link light is solid on both the phone and the switch port, but the phone's display remains dark and it does not power on. The technician has verified the cable with a tester and it passes. The switch port is enabled and configured for the same VLAN as other working phones. Which of the following is the MOST likely cause?
Quick Answer
This question is built to make you separate data connectivity from power delivery, since PoE switches provide both over the same cable but manage them through different mechanisms. A solid link light on both ends confirms that Layer 1 data connectivity is fully working, and the technician already validated the cable, ruled out a VLAN mismatch, and confirmed the port itself is enabled, which eliminates the usual data-path suspects. What remains is that the phone's display stays dark and it never powers on at all, a symptom that has nothing to do with data and everything to do with power delivery. PoE switches have a finite total power budget shared across every connected device, and if the combined demand from all currently powered devices reaches that ceiling, the switch can and will decline to energize an additional port, even though that port is otherwise correctly configured and has a valid link. This produces the exact combination described here: full Layer 1 connectivity with no power delivered. Whenever a PoE device shows a working link light but never powers on, and the data-side configuration has already been confirmed correct, think about the switch's overall power budget rather than continuing to troubleshoot cabling, VLANs, or port settings.
⚠ Common exam trap
The trap here is that candidates see a solid link light and assume the port is fully functional, overlooking that PoE power allocation is independent of data link status; CompTIA often tests this by showing a working link light but a non-powered device to trick you into focusing on Layer 2 or Layer 3 issues.
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's PoE power budget has been exceeded.
A PoE-managed switch allocates power from a finite budget. If the total power draw of all connected devices exceeds the switch's PoE power budget, the switch may deny power to additional devices even if the port is administratively enabled and the link is up. The solid Ethernet link light indicates Layer 1 connectivity, but the phone's dark display and lack of power suggest it is not receiving PoE, which is consistent with the switch having exhausted its PoE budget.
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's PoE power budget has been exceeded.
Why this is correct
A solid link light confirms the data pair is up, so the cable and port are fine. PoE negotiation happens on the spare pairs before power is delivered; if the switch's total power budget is exhausted, it withholds power, leaving the phone dark despite a live Ethernet link.
- ✗
The phone's MAC address is incorrect.
Why it's wrong here
MAC addresses are burned into the phone's NIC and cannot be misconfigured in a way that kills power; a wrong MAC would affect layer-2 forwarding, not PoE. It is tempting because MAC problems do cause connectivity faults, but the dark display points to insufficient power from the switch port.
- ✗
The cable is a crossover cable.
Why it's wrong here
Modern switches and phones implement auto-MDI-X, so a crossover cable still establishes the link shown by the solid link lights. It is tempting because crossover cables historically caused link failures, but the symptom here is no power, not no link, so PoE delivery is the issue.
- ✗
The switch port is in a different subnet.
Why it's wrong here
A subnet mismatch would not extinguish PoE power, since the phone draws power at layer 1 before any IP addressing occurs. It is tempting because VLAN and subnet errors do break VoIP connectivity, but here the link is up and the display is dark, indicating absent power delivery.
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Same concept, more angles
1 more way this is tested on 220-1101
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A technician installs a new VoIP desk phone for a user. The phone is connected to an existing wall jack using a Cat5e patch cable. The switch port on the other end shows a solid link light, but the phone does not power on. The technician verifies that the wall jack is wired correctly and the cable passes a continuity test. Which of the following is the MOST likely reason the phone is not receiving power?
medium- A.The switch port is administratively disabled
- B.The patch cable is a crossover cable
- ✓ C.The network switch does not support Power over Ethernet (PoE)
- D.The wall jack is wired with T568A instead of T568B
Why C: The phone is not powering on despite a solid link light and a correctly wired, continuity-tested cable. This indicates the switch is providing a data link but not delivering power. The most likely cause is that the network switch does not support Power over Ethernet (PoE), which is required to supply DC power over the same Cat5e cable to the VoIP phone.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1101 practice question is part of Courseiva's free CompTIA 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 220-1101 exam.