Voice VLAN: Phone Has No Service
A desk phone has link and power but no voice path. Diagnose the access port's voice VLAN and fix it yourself, live, right below.
Symptom
The PC works through the phone, but the phone never joins its voice network. Restore the tagged voice path without breaking the data VLAN.
Your mission
Prove the cause, then fix it
Format
Guided CLI investigation
Investigate before you configure.
Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.
Observe the symptom and link state.
Diagnose by comparing the protocol evidence.
Verify the expected device state and confirm the original symptom is resolved.
Console access: SW1
This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.
Need a hint?
Reveal the root cause when you're ready.
Try the investigation first, then use this as your escape hatch, not a dead end.
Lab debrief
Turn the session into a repeatable troubleshooting pattern.
Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.
The problem
An IP phone with a PC connected through it is plugged into SW1 GigabitEthernet0/1. The PC reaches the data network normally, but the phone never receives voice service even though VLAN 50 exists on the switch.
What you'll practice
- Distinguish the data VLAN from the voice VLAN on one access port
- Read operational switchport state and compare it with running configuration
- Verify that the required voice VLAN exists
- Configure a voice VLAN without replacing the data VLAN
- Apply and verify PortFast on an end-device port
The topology
One access switch connects an IP phone with a PC daisy-chained through it. GigabitEthernet0/1 should carry untagged data in VLAN 10 and phone-tagged traffic in voice VLAN 50.
Commands to run yourself
The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.
show interfaces GigabitEthernet0/1 switchportshow running-config interface GigabitEthernet0/1show vlan briefTopology diagram
Frequently asked
Is a voice VLAN port a trunk?
Not in the normal switchport-mode sense. The port remains an access port for the PC's untagged data while the phone tags voice frames with the separately advertised voice VLAN.
Why can the PC work while the phone fails?
The two devices use different Layer 2 paths on the same cable. A correct access VLAN proves only the untagged data side; it says nothing about whether a voice VLAN was assigned for the phone's tagged frames.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.