Voice VLAN

1. Purpose of Voice VLAN

A Voice VLAN is designed to separate and prioritize VoIP (Voice over IP) traffic on a network. By isolating voice traffic from data traffic, it ensures high-quality audio communication, reducing latency, jitter, and packet loss.

Initial Configuration Challenges

Traditionally, connecting a computer and an IP phone to a switch required two cables:

  1. One cable connecting the computer to the switch.
  2. Another cable connecting the IP phone to the switch.
Voice VLAN Old Topology

This approach has several disadvantages:

  • It requires additional cabling, increasing installation complexity.
  • It consumes two switch ports per workstation, reducing port availability.

2. Understanding Voice VLAN

To address these challenges, modern IP phones (such as Cisco models) include an integrated three-port switch:

  1. One port connects the phone to the switch.
  2. One port connects the computer to the phone.
Voice VLAN Topology

This setup enables shared cabling while logically separating traffic using VLANs:

  • The data VLAN handles computer traffic.
  • The Voice VLAN (also called Auxiliary VLAN) manages IP phone traffic.

When connected, the switch port acts as a trunk for the phone, forwarding untagged traffic (from the computer) and tagged traffic (from the phone). Only the data and Voice VLANs are allowed on this trunk.

Voice VLAN High Priority

3. Configuring Voice VLAN

Configuration Steps

Voice VLAN Topology 1

Let’s configure a Voice VLAN using VLAN 10 for data and VLAN 20 for voice:

  1. Create the VLANs:
SW1(config)# vlan 10
SW1(config-vlan)# name DATA
SW1(config-vlan)# exit

SW1(config)# vlan 20
SW1(config-vlan)# name VOICE
SW1(config-vlan)# exit

4. Configure the switch port:

SW1(config)# interface GigabitEthernet 0/0
SW1(config-if)# switchport mode access
SW1(config-if)# switchport access vlan 10
SW1(config-if)# switchport voice vlan 20
SW1(config-if)# exit
  • VLAN 10 is used for computer traffic.
  • VLAN 20 is used for voice traffic.
  • Enable VLAN Discovery:
    • Cisco IP phones use CDP (Cisco Discovery Protocol) to learn which VLANs to use.
    • Other phones may use LLDP-MED (Link Layer Discovery Protocol – Media Endpoint Discovery) for this purpose.

4. Verifying and Troubleshooting Voice VLAN

Verify VLAN Configuration

Use the following command to verify the switch port settings:

SW1# show interfaces GigabitEthernet 0/0 switchport
Name: Gi0/0
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: Off
Access Mode VLAN: 10 (DATA)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: 20 (VOICE)

Check Allowed VLANs

Even though the interface operates as a trunk, the allowed VLANs are limited to the data and Voice VLANs:

SW1# show interfaces GigabitEthernet 0/0 trunk
Port        Mode             Encapsulation  Status        Native vlan
Gi0/0       off              negotiate      not-trunking  1

Port        Vlans allowed on trunk
Gi0/0       10,20

Port        Vlans allowed and active in management domain
Gi0/0       10,20

Port        Vlans in spanning tree forwarding state and not pruned
Gi0/0       10,20

5. Conclusion

With this configuration, you can efficiently connect both a computer and an IP phone using a single port and cable, while ensuring logical separation of traffic. The Voice VLAN setup improves network performance and maintains call quality by prioritizing voice traffic.

This approach simplifies network management, conserves switch ports, and enhances the user experience in environments where VoIP is critical.

VTP