N10-009 Question 152
Single answer2.2 Given a scenario, configure switching technologies and features.A network administrator is replacing an unmanaged switch in a small office with a managed Layer 2 switch. The office has IP phones with PCs connected through the phones, and users report poor call quality whenever large file transfers occur. The administrator wants to prioritize voice traffic without redesigning the IP addressing scheme or adding new physical switches. Which configuration should the administrator implement to best meet this requirement?
- A
Configure a voice VLAN on the switch ports used by the phones and trust/apply QoS markings for voice traffic
- B
Enable port security on the user-facing switch ports and limit each port to two MAC addresses
- C
Create an LACP EtherChannel between the switch and the phones to increase available bandwidth
- D
Configure STP portfast on the user-facing access ports to reduce the time required for links to become active
Show answer and explanation
Correct answer: A
Explanation
The best solution is to configure a voice VLAN and QoS on the access ports serving the IP phones. In many enterprise switch deployments, an access port can carry both untagged data traffic for the attached PC and tagged voice traffic for the phone by using a dedicated voice VLAN. QoS then prioritizes voice based on Layer 2 CoS or Layer 3 DSCP markings, reducing latency, jitter, and packet loss during congestion. This is a standard best practice in converged voice/data networks and is supported by major switch vendors such as Cisco, Aruba, and Juniper in their campus switching design guidance. The other options are legitimate switch features, but they address different needs: port security protects access ports, EtherChannel increases bandwidth and redundancy between supported links, and PortFast speeds STP convergence for edge devices. None of those directly solves poor VoIP performance caused by contention with large data transfers.
- A. Correct.
Correct. A voice VLAN allows the switch to logically separate voice traffic from regular data traffic on the same physical access port, which is a common deployment when an IP phone has a PC connected through it. Combined with QoS, the switch can prioritize delay-sensitive voice traffic over bulk data transfers. This directly addresses the call-quality issue without changing the overall IP design or adding hardware.
- B. Incorrect.
Incorrect. Port security can help prevent unauthorized devices or MAC flooding attacks, and allowing two MAC addresses is common for a port with both a phone and a PC. However, port security does not prioritize traffic or reduce latency and jitter for voice calls, so it does not solve the stated problem.
- C. Incorrect.
Incorrect. LACP EtherChannel is used to bundle multiple physical links between compatible devices such as switches or servers into a single logical link. It is not used between a switch and individual IP phones in this scenario. Even if more bandwidth were available elsewhere, the key issue described is traffic prioritization, which EtherChannel alone does not provide.
- D. Incorrect.
Incorrect. Portfast allows access ports to transition to forwarding more quickly, which helps endpoints connect faster after link-up and can reduce delays during boot or reconnection. However, it does not prioritize voice traffic or address voice quality degradation during periods of heavy data transfer.