N10-009 Question 328
Single answerBasic networking device commands: show mac-address-table, show route, show interface, show config, show arp, show vlan, show powerA technician is troubleshooting a report that a newly installed IP phone is not powering on when connected to a managed switch. The switch supports Power over Ethernet (PoE), and other devices on the same switch are working normally. The technician wants to verify whether the switch is detecting the phone and whether PoE is being delivered on that specific port. Which command should the technician use first?
- A
show power
- B
show mac-address-table
- C
show interface
- D
show arp
Show answer and explanation
Correct answer: A
Explanation
This question tests practical command selection during switch troubleshooting. When the issue specifically involves a powered device such as an IP phone not turning on, the technician should start with the command that directly checks Power over Ethernet status. On managed switches, show power is commonly used to review PoE availability, per-port power draw, and whether the switch has detected a powered device. Commands such as show mac-address-table, show interface, and show arp can all be useful in broader troubleshooting, but they do not directly answer the key question in this scenario: is the switch delivering power to the port? This aligns with common vendor documentation and standard troubleshooting practice of starting with the most symptom-specific command before moving to Layer 2 and Layer 3 verification.
- A. Correct.
Correct. The
show powercommand is the best first choice when troubleshooting a PoE issue on a switch. It is used to verify PoE status, power allocation, and whether a port is providing power to a connected powered device such as an IP phone or wireless access point. In this scenario, the primary symptom is that the phone is not powering on, so confirming PoE delivery is the most direct and relevant first step. - B. Incorrect.
Incorrect. The
show mac-address-tablecommand displays learned Layer 2 MAC addresses and the switch ports associated with them. This can help determine whether the switch has seen traffic from a device, but if the phone is not powering on at all, it may never transmit frames and therefore may not appear in the MAC table. This command does not directly confirm whether PoE is being supplied. - C. Incorrect.
Incorrect. The
show interfacecommand can provide useful information about interface status, errors, and whether a port is up or down. However, it is not the most direct command for verifying PoE delivery. A port could appear physically connected while still not providing the power needed by the IP phone. It is a reasonable secondary troubleshooting step, but not the best first command here. - D. Incorrect.
Incorrect. The
show arpcommand displays the ARP table, which maps IPv4 addresses to MAC addresses. ARP information is relevant only after a device is powered on, connected, and communicating at Layer 3. Since the problem is that the phone is not even powering on, ARP is too far up the stack to help as an initial troubleshooting step.