CCNA Switching and Network Access Practice Question
Exhibit
SW1# show running-config interface GigabitEthernet0/1 Building configuration... Current configuration : 128 bytes ! interface GigabitEthernet0/1 switchport mode access switchport access vlan 10 spanning-tree portfast end SW1#
A network administrator connects an IP phone to interface Gi0/1 and a PC to the phone's PC port. Users report that the phone cannot register with the call manager and the PC does not obtain an IP address. The administrator verifies the configuration shown in the exhibit. What is the most likely cause of the problem?
⚠ Common exam trap
Cisco often tests the distinction between access VLAN and voice VLAN, and the trap here is that candidates assume the access VLAN alone is sufficient for both voice and data, overlooking the mandatory 'switchport voice vlan' command required for IP phone operation.
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 voice VLAN is not configured on the interface.
The voice VLAN is not configured on the interface. For an IP phone to communicate with the call manager, the switchport must be configured with a voice VLAN (e.g., 'switchport voice vlan 20') to separate voice traffic from data traffic. Without this command, the phone cannot tag its frames with the appropriate VLAN ID, preventing registration. Additionally, the PC connected to the phone's PC port relies on the phone to pass untagged data traffic, but without the voice VLAN, the phone may not properly negotiate or forward traffic, causing the PC to fail to obtain an IP address.
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 should be changed to trunk to allow tagged voice traffic.
Why it's wrong here
Changing the switchport mode to trunk would not resolve the issue because the PC and phone both require untagged access to a single VLAN for data and a separate voice VLAN, whereas trunking would expect the switch to carry multiple tagged VLANs and would strip the native VLAN’s tag, breaking the PC’s untagged traffic. This option is tempting because trunk ports are used to carry voice VLAN traffic for IP phones in some designs, but here the correct solution must enable the switch to place the phone’s traffic into a dedicated voice VLAN while keeping the PC in an access VLAN, which trunk mode does not achieve.
- ✓
The voice VLAN is not configured on the interface.
Why this is correct
In a typical IP phone deployment, the access port must be configured with both an access VLAN for the PC and a separate voice VLAN for the phone. The exhibit shows only `switchport access vlan 10` but lacks `switchport voice vlan`, so the switch cannot identify tagged voice frames from the phone, and the phone fails to obtain an IP address or register. Adding the voice VLAN command instructs the switch to accept 802.1Q-tagged traffic from the phone while keeping PC data untagged on the access VLAN, which resolves the registration failure.
- ✗
The spanning-tree portfast command is blocking the port from forwarding.
Why it's wrong here
PortFast does not impede forwarding; rather, it transitions a switchport from blocking to forwarding state immediately after link-up, skipping the Spanning Tree listening and learning states. This actually reduces the delay before traffic is forwarded, so it could not be the cause of the phone's failure to register. If PortFast were blocking the port, the PC and phone would both experience a complete loss of connectivity, not just the voice service.
- ✗
The access VLAN 10 is incorrect for the data subnet.
Why it's wrong here
The exhibit does not reveal the intended VLAN numbering, but the core symptom (phone cannot register) points to a voice VLAN issue. An incorrect data VLAN would affect the PC's IP assignment but not the phone's registration process.
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 voice VLAN is not configured on the interface.Correct answer▾
Why this is correct
In a typical IP phone deployment, the access port must be configured with both an access VLAN for the PC and a separate voice VLAN for the phone. The exhibit shows only `switchport access vlan 10` but lacks `switchport voice vlan`, so the switch cannot identify tagged voice frames from the phone, and the phone fails to obtain an IP address or register. Adding the voice VLAN command instructs the switch to accept 802.1Q-tagged traffic from the phone while keeping PC data untagged on the access VLAN, which resolves the registration failure.
✗The switchport mode should be changed to trunk to allow tagged voice traffic.Wrong answer — click to see why▾
Why this is wrong here
Cisco best practice for IP phone + PC is an access port with switchport voice vlan, not trunk mode. Trunk mode is more complex and can cause VLAN hopping if misconfigured.
✗The spanning-tree portfast command is blocking the port from forwarding.Wrong answer — click to see why▾
Why this is wrong here
Portfast enables faster convergence, not a blocking action. A blocked port would show no link or constant amber LED, not intermittent connectivity.
✗The access VLAN 10 is incorrect for the data subnet.Wrong answer — click to see why▾
Why this is wrong here
Even if VLAN 10 were wrong, the phone’s registration issue would persist because the voice VLAN is still missing. The problem has two symptoms; the voice VLAN omission explains both.
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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Key term
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
Key term
IP address
An IP address is a unique numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.
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JA
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.