Cisco Certified Support Technician (CCST) NetworkingNetwork AccessHard
A network administrator is troubleshooting an issue where a newly deployed Cisco IP Phone is not receiving an IP address from the DHCP server on a switch port configured for both data and voice VLANs. The phone is connected to the switch, and a computer is connected to the phone's built-in switch port. The computer receives an IP address successfully from the data VLAN, but the phone does not get an IP address for its voice VLAN. Which of the following is the MOST likely cause?
- AThe voice VLAN is not correctly configured or allowed on the switch port.
- BThe computer's MAC address is blocking the phone's DHCP request.
- CThe DHCP server is down or unreachable for the data VLAN.
- DThe switch port is configured as an access port instead of a trunk port.
Show answer & explanationAnswer & explanation
Correct answer: A. The voice VLAN is not correctly configured or allowed on the switch port.
Cisco IP Phones use CDP (Cisco Discovery Protocol) or LLDP-MED (Media Endpoint Discovery) to learn the voice VLAN ID from the switch. If the voice VLAN is not correctly configured (e.g., 'switchport voice vlan [vlan-id]') or is not allowed on the port, the phone will not be able to obtain an IP address for its voice VLAN, even if the data VLAN is working for the connected PC.
Why the other options are wrong
- B. A computer's MAC address would not block a phone's DHCP request in this manner. MAC address issues typically involve port security or MAC address tables, not selective DHCP blocking between devices on the same port.
- C. The scenario states the computer receives an IP address successfully from the data VLAN, indicating the DHCP server is likely reachable for at least the data VLAN.
- D. For an IP phone with a connected PC, the switch port must be configured as an access port, with a data VLAN and a voice VLAN. A trunk port would be incorrect for an end device.
Voice VLAN Configuration
Voice VLANs are used to separate IP phone traffic from data traffic on the same switch port, ensuring QoS for voice and simplifying network management.
- IP phones typically have a built-in 3-port switch (one for switch, one for phone, one for PC).
- Uses 'switchport voice vlan [vlan-id]' command on Cisco switches.
- Phone learns voice VLAN ID via CDP or LLDP-MED.
- Ensures voice traffic is tagged with the correct VLAN ID.
Memory trick: Voice VLAN Missing, Phone's Not Listening