Dedicated VLANs for VoIP traffic with QoS prioritization and separation from data traffic
Voice VLAN
Voice VLANs are separate Layer 2 broadcast domains specifically designed for IP phone traffic - this segregation provides Quality of Service (QoS) prioritization and security isolation from data traffic
Operates by allowing a single switch port to carry both data VLAN (untagged) and voice VLAN (tagged) traffic simultaneously
IP phones act as mini-switches with built-in 3-port switch functionality - one port connects to switch, one to PC, and internal port for phone operations
Voice traffic receives 802.1Q VLAN tagging while PC data remains untagged on the native/access VLAN
Switch port configuration uses switchport voice vlan <vlan-id> command in addition to standard access VLAN configuration
Key Benefits
QoS Implementation: Voice traffic can be prioritized using Class of Service (CoS) markings in 802.1Q headers
Security Separation: Voice and data traffic isolated into different broadcast domains
Bandwidth Management: Dedicated VLAN allows for traffic shaping and monitoring
Simplified Cabling: Single cable run supports both phone and PC connectivity
Configuration Components
CDP/LLDP Discovery: Switch advertises voice VLAN information to compatible IP phones
Power over Ethernet (PoE): Often deployed alongside voice VLANs for phone power delivery
Trust Boundaries: Switch trusts CoS markings from IP phones but not from connected PCs
Traffic Type
VLAN Tagging
Priority
Typical VLAN ID Range
Voice
802.1Q Tagged
High (CoS 5)
100-199
Data
Untagged (Native)
Normal (CoS 0)
Any
Vocabulary
Voice VLAN: Dedicated VLAN for IP telephony traffic with QoS prioritization
Auxiliary VLAN: Cisco term for voice VLAN configuration on switch ports
CoS (Class of Service): Layer 2 QoS marking (0-7) carried in 802.1Q VLAN tags
Trust Boundary: Point in network where QoS markings are trusted or remarked
Notes
Voice VLANs require 802.1Q trunking knowledge - phones tag voice traffic but leave data untagged
Default voice VLAN behavior varies by vendor - Cisco uses auxiliary VLAN concept while others may differ
IP phones typically receive VLAN assignment via CDP/LLDP advertisements from switch
Common deployment uses separate IP subnets for voice and data VLANs (e.g., 10.1.100.0/24 for voice, 10.1.10.0/24 for data)