Export PDFWhat Is Voice VLAN?
Voice VLAN is a technology that transmits voice data.
Why Do We Need Voice VLAN?
Data, voice, and video services are often transmitted simultaneously over a network. Voice services, in particular, require a higher forwarding priority than data or video services. When bandwidth is limited, voice data must have transmission preference over other types of data. This can be ensured by configuring a voice VLAN on the switch to transmit voice data and setting QoS parameters in the voice VLAN so that voice data is given preference when congestion occurs.
What Are the Application Scenarios of Voice VLAN?
The following figure shows PCs and IP phones connecting to the Internet through switches. Because the voice service is sensitive to delay and jitter, the priority of voice data flows needs to be increased so that they can be preferentially forwarded when congestion occurs.
Application scenario of voice VLAN
- Configure MAC address-based voice VLAN when voice packets are untagged or tagged with VLAN 0.
- Configure VLAN ID-based voice VLAN when IP phones can obtain voice VLAN information on the switch.
How Does Voice VLAN Work?
Voice data flows must be identified first so that measures can be taken to improve their transmission priority.
The switch configured with voice VLAN can identify voice data flows in either of the following modes:
Based on source MAC addresses in received packets
The switch determines whether a data packet received by an interface is a voice data packet based on the source MAC address field in the packet. When the source MAC address in a data packet matches the organizationally unique identifier (OUI) that has been preconfigured in the system for a voice device, the switch considers the data packet as a voice data packet. Such a preconfigured OUI applies to the scenario where IP phones send untagged voice packets.
Based on VLAN IDs in received packets
It takes a long time to configure OUIs for IP phones if many of them are connected to the switch. Instead, you can configure the switch to increase the priority of voice data packets based on VLAN IDs. The switch then determines whether a data packet received by an interface is a voice data packet based on the VLAN ID in the packet. When the VLAN ID in a data packet matches the voice VLAN ID configured in the system, the switch considers the data packet as a voice data packet. For this mode to work, the IP phones must be able to obtain the information about the voice VLAN configured on the switch. When many IP phones are connected to the switch, the configuration can be simplified.
The solutions above simplify configurations. Whether or not the voice packets sent by IP phones are VLAN-tagged does not impact the switch's ability to identify voice packets based on MAC addresses and VLAN IDs. Therefore, if IP phones send tagged voice packets, you can simply configure the voice VLAN function based on VLAN IDs, simplifying the configurations. However, if the packets are untagged, OUIs must be configured to distinguish voice packets from data packets.
MAC Address-based Voice VLAN
- OUI
An OUI refers to the first 24 bits of a MAC address and can be used to identify a MAC address range. The Institute of Electrical and Electronics Engineers (IEEE) assigns a unique OUI to each vendor. In addition, each vendor assigns another 24 bits to form a 48-bit MAC address. Therefore, voice packets from IP phones can be identified by the OUIs allocated to IP phone vendors based on MAC address ranges requested by IP phone vendors.
An OUI in voice VLAN is different from a standard OUI. It is user-configured and can use a mask, whose length is not restricted to 24 bits but can be instead specified by the user. The OUI is the result of the AND operation between the MAC address and mask in the voice-vlan mac-address command.
- Implementation
The following figure illustrates MAC address-based voice VLAN. After receiving untagged packets from the PC and IP phone, the switch processes the packets as follows: If the source MAC address matches the OUI configured on the switch (that is, the result of the AND operation between the source MAC address and the configured OUI mask is the same as the OUI), the switch adds the voice VLAN tag to the packets and increases the packet priority, ensuring that the voice packets are sent preferentially. If the source MAC address does not match the configured OUI, the switch adds the default VLAN tag (PVID) to the packets.
MAC address-based voice VLAN
VLAN ID-based Voice VLAN
After receiving packets from the PC and IP phone, the switch determines whether the VLAN IDs in the packets match the configured voice VLAN ID. If they match, the switch considers data as voice data and increases the priority. The switch adds the VLAN tag of the PVID to untagged packets from the PC. When VLAN ID-based voice VLAN is configured, the IP phone must be able to obtain voice VLAN information from the switch.
VLAN ID-based voice VLAN
- In the preceding figure, when the IP phone goes online, it sends an LLDPDU to the switch to obtain the information about the voice VLAN configured on the switch.
- After receiving the LLDPDU, the switch encapsulates voice VLAN information in the LLDPDU and sends it to the IP phone.
- After receiving the LLDPDU carrying the voice VLAN information, the IP phone sends tagged voice packets.
- The switch receives the tagged voice packets. If the VLAN ID in the packets matches the voice VLAN ID configured on the switch, the switch increases the priority of the packets and forwards them.
After receiving untagged packets, the switch still sends them in the VLAN specified by the PVID. In this way, the switch sends voice packets preferentially when congestion occurs.
- Author： Meng Xianhai, Li Qiang
- Updated on： 2025-07-18
- Views： 1397
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