N10-009 Question 156
Single answerA network administrator is deploying new IP phones with built-in two-port switches so that each user's PC connects through the phone to an access switch. The switch port must carry both user data and voice traffic. After deployment, PCs can reach the network, but phones fail to register with the call manager. The switch uplinks and DHCP scopes are already confirmed to be working, and the phone vendor documentation states that the phone expects untagged traffic for the data network and tagged traffic for the voice network. Which switch interface configuration is the BEST way to resolve the issue?
- A
Configure the port as an access port in the data VLAN and assign a voice VLAN so the switch sends voice traffic with 802.1Q tags while keeping PC traffic untagged
- B
Configure the port as a trunk port and manually set the native VLAN to the voice VLAN so the phone receives untagged voice traffic
- C
Enable jumbo frames on the interface so the phone can pass both voice and data traffic without fragmentation
- D
Disable spanning tree on the interface to prevent the phone from blocking tagged voice frames
Show answer and explanation
Correct answer: A
Explanation
The key detail is that the phone expects data traffic to be untagged and voice traffic to be tagged. That matches the common access-port-plus-voice-VLAN design used on enterprise switches. In this model, the switchport remains in the data VLAN for the attached PC, and a separate voice VLAN is configured so the phone tags its voice frames with 802.1Q. This allows the phone and downstream PC to share one physical interface while keeping voice and data logically separated for QoS, security, and policy enforcement. By contrast, confusing the native VLAN with the voice VLAN is a frequent implementation error because the native VLAN on an 802.1Q trunk is carried untagged. Best practices from major switch vendors document using an access VLAN plus a dedicated voice VLAN for IP phone deployments rather than making the user edge port a general trunk solely to support the phone.
- A. Correct.
Correct. This is the standard enterprise switch configuration for an IP phone with a downstream PC. The access VLAN carries normal user data as untagged traffic for the attached computer, while the voice VLAN allows the phone to place voice traffic into a separate VLAN using 802.1Q tagging. Many switch platforms support a dedicated voice VLAN feature specifically for this design.
- B. Incorrect.
Incorrect. Making the port a trunk and setting the native VLAN to the voice VLAN reverses the expected tagging behavior from the scenario. The vendor documentation states the phone expects untagged traffic for data and tagged traffic for voice. On an 802.1Q trunk, the native VLAN is typically untagged, so placing voice on the native VLAN would cause voice to be untagged instead of tagged. This is a common misconception when mixing native VLAN and voice VLAN concepts.
- C. Incorrect.
Incorrect. Jumbo frames increase the MTU, typically for storage or high-throughput workloads, but they are not required for standard VoIP registration and would not fix a VLAN tagging mismatch. Voice traffic uses relatively small packets, so MTU changes are not the likely cause in this scenario.
- D. Incorrect.
Incorrect. Spanning Tree Protocol helps prevent Layer 2 loops. Disabling spanning tree does not solve incorrect VLAN tagging and can create switching loops if another path is introduced. While edge features such as PortFast may be appropriate on phone/user access ports, fully disabling spanning tree is not the best practice and is unrelated to the symptom described.