CCNA Switching and Network Access Practice Question
Exhibit
interface GigabitEthernet1/0/24 description VoIP and Desktop switchport mode access switchport access vlan 10 switchport voice vlan 20 spanning-tree portfast !
A network administrator has configured a switch port to support a VoIP phone and a desktop PC. Users report that the desktop PC cannot obtain an IP address via DHCP, while the VoIP phone registers successfully. The switch port is up/up, and the desktop is connected to the phone's PC port. What is the most likely cause of the issue?
⚠ Common exam trap
Cisco often tests the misconception that the PC uses the voice VLAN or that the port must be a trunk, when in reality the phone handles the VLAN separation internally and the switch port remains an access port with a separate voice VLAN.
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 switchport access vlan should be changed to the data VLAN to match the subnet expected by the desktop.
The desktop PC obtains its IP address from the data VLAN, but the switch port's access VLAN is likely misconfigured to the voice VLAN. Changing the access VLAN to the correct data VLAN places the PC in the proper subnet and allows DHCP to function. Option A is unnecessary because a voice-access port does not need to be a trunk. Option C would break the VoIP phone without fixing the PC's VLAN assignment. Option D is irrelevant; PortFast speeds up STP convergence and does not block DHCP.
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 switchport mode access should be changed to switchport mode trunk to allow both vlans.
Why it's wrong here
Changing the switchport mode from access to trunk would force the IP phone to 802.1Q-tag all traffic, including data from the desktop, which is not how typical Cisco IP phones operate. The phone expects the desktop to send untagged frames over the PC port, and the switchport should remain in access mode with a configured voice VLAN. A trunk link would put untagged desktop traffic on the native VLAN, likely mismatching the intended data subnet, so this change would not resolve the DHCP failure.
- ✓
The switchport access vlan should be changed to the data VLAN to match the subnet expected by the desktop.
Why this is correct
In a typical IP phone daisy-chain topology, the desktop connects to the phone's PC port, and the switch sends all untagged desktop frames into the port's configured access VLAN. If that access VLAN is incorrectly set to, say, the native VLAN or a different number, the desktop's DHCP DISCOVER will not arrive at the DHCP server for its expected data subnet, leaving it without an IP address. Correctly changing the switchport access vlan to the data VLAN ensures the desktop's untagged traffic is placed in the proper L2 domain, allowing the DHCP server to respond with an appropriate address.
- ✗
The switchport voice vlan should be removed because the desktop cannot use it.
Why it's wrong here
The voice VLAN is dedicated to the IP phone's internal switch, carrying tagged VoIP traffic between the phone and the call control system. Removing it would leave the phone without a valid network segment for voice traffic, preventing it from registering or receiving an IP address. The desktop does not use the voice VLAN at all; it relies on the access VLAN, so deleting the voice VLAN would break the phone while doing nothing to fix the desktop's DHCP problem.
- ✗
The spanning-tree portfast should be disabled to prevent DHCP delays.
Why it's wrong here
Portfast is a Cisco feature that immediately moves an access port to the forwarding state, eliminating the 30-second listening and learning phase of Spanning Tree Protocol. Disabling portfast would actually increase DHCP delays because the desktop's DHCP DISCOVER would be held back until STP converges, and it could even cause the port to block frames temporarily. The real issue is that the desktop is on the wrong access VLAN, so its DHCP packets are not reaching the correct DHCP server, which has nothing to do with STP timing.
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.
✓The switchport access vlan should be changed to the data VLAN to match the subnet expected by the desktop.Correct answer▾
Why this is correct
In a typical IP phone daisy-chain topology, the desktop connects to the phone's PC port, and the switch sends all untagged desktop frames into the port's configured access VLAN. If that access VLAN is incorrectly set to, say, the native VLAN or a different number, the desktop's DHCP DISCOVER will not arrive at the DHCP server for its expected data subnet, leaving it without an IP address. Correctly changing the switchport access vlan to the data VLAN ensures the desktop's untagged traffic is placed in the proper L2 domain, allowing the DHCP server to respond with an appropriate address.
✗The switchport mode access should be changed to switchport mode trunk to allow both vlans.Wrong answer — click to see why▾
Why this is wrong here
The current configuration uses access mode with voice vlan, which is correct for a phone+PC setup. Trunk mode is unnecessary and could break the phone's untagged traffic expectations.
✗The switchport voice vlan should be removed because the desktop cannot use it.Wrong answer — click to see why▾
Why this is wrong here
The voice vlan is correctly configured for the phone; the problem is with the data vlan assignment for the desktop.
✗The spanning-tree portfast should be disabled to prevent DHCP delays.Wrong answer — click to see why▾
Why this is wrong here
Portfast is beneficial for host ports; disabling it would worsen the issue by introducing STP convergence delays.
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?”
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Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
Bridge Protocol Data Unit
A Bridge Protocol Data Unit (BPDU) is a special message that network switches exchange to detect and prevent loops in an Ethernet network.
Key term
PortFast
PortFast is a Cisco switch feature that immediately brings a port into the forwarding state, bypassing the normal Spanning Tree Protocol (STP) listening and learning phases, so that devices connected to that port can start communicating right away.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco 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 200-301 exam.