OBJECTIVE
The purpose of this post is to present an introduction to Voice VLAN.
Introduction of Voice VLAN
Voice data and non-voice data are often transmitted on the same network, but voice data requires a higher priority than other service data during transmission to shorten the packet delay and reduce packet loss during transmission.
In the voice VLAN system, voice data is differentiated from other data through Access Control Lists (ACLs) to ensure preferential transmission, and transmission quality is ensured through Quality of Service (QoS).
An interface enabled with voice VLAN determines whether incoming data is voice data by examining the data packet's source MAC addresses. If the source MAC address matches the Organizationally Unique Identifier (OUI), data with that source MAC address is considered voice data. The interface receiving voice data is then automatically added to the voice VLAN, effectively simplifying configurations and allowing users to manage voice data more conveniently.
As shown in Figure 1, High Speed Internet (HSI) services, Voice over IP (VoIP) services, and Internet Protocol Television (IPTV) services are all transmitted to Device. To differentiate voice data from other data, VoIP traffic is isolated through different VLANs and is assigned a higher priority to ensure voice quality. Therefore, when voice VLAN is configured on the Device, the Device adds a pre-configured VLAN ID and assigns a higher priority to VoIP traffic.
Figure 1 - Typical voice VLAN networking diagram
On different interfaces of a Device, you can specify multiple VLANs as voice VLANs. However, you can specify only one VLAN as a voice VLAN on an interface.
OUI Address of Voice VLAN
The OUI indicates a MAC address segment.
You can perform an AND operation between a 48-bit MAC address and a mask to obtain the OUI.
The length of all 1s in the mask determines the number of matched bits between a device's MAC address and the OUI. For example, if the specified MAC address is 1-1-1 and the mask is FFFF-FF00-0000, the OUI is 0001-0000-0000. In this example, if the first 24 bits of the MAC address of the device match the first 24 bits of the OUI, the interface enabled with voice VLAN considers the data from the access device as voice data, and the device as a voice device.
Mode Used to Add an Interface to a Voice VLAN
An interface can be added to the voice VLAN in manual mode or auto mode.
Auto mode
An interface enabled with voice VLAN identifies voice data flows according to the source MAC address of the received data packets. If the source MAC address of a flow matches the OUI of the voice VLAN, the flow is considered as a voice data flow.
The system automatically adds the interface receiving voice data flows to the voice VLAN and maintains the number of interfaces within the voice VLAN using the aging mechanism.
Before the aging timer expires:
If the interface does not receive any voice data packet from a voice device, the interface is automatically deleted from the voice VLAN.
If the interface receives voice data packets from the voice device again, the interface is automatically added to the voice VLAN.
In auto mode, the interface enabled with voice VLAN forwards both voice data packets and service data packets. Figure 2 shows a voice VLAN in auto mode.
Figure 2 - Networking diagram of a voice VLAN in auto mode
Manual mode
In manual mode, the interface connected to an IP phone is manually added to the voice VLAN. The device checks whether the source MAC address of each packet matches the OUI. If the source MAC address of a packet matches the OUI, the device delivers ACL rules and changes the priority of the packet. The administrator adds an interface to or deletes it from the voice VLAN manually.
In manual mode, the interface enabled with voice VLAN forwards only voice data packets. Figure 3 shows a voice VLAN in manual mode.
Figure 3 - Networking diagram of a voice VLAN in manual mode
Different interfaces can be added to voice VLANs in different modes, each of which are independent of each other.
Working Mode of a Voice VLAN
| Mode | Description | Application Scenario |
| Security mode | An interface enabled with voice VLAN checks whether the source MAC address of each packet entering the voice VLAN matches the OUI.
| Security mode is used when multiple services (HSI, VoIP, and IPTV) are accessed on a Layer 2 network through a single interface, and the interface transmits only voice data. The security mode can protect the voice VLAN against the attacks by invalid packets, but checking packets occupies certain system resources. |
| Normal mode | The interface enabled with voice VLAN can transmit both voice data and service data, and is vulnerable to attacks by invalid packets. | The normal mode is used when multiple services (HSI, VOIP, and IPTV) are transmitted to a Layer 2 network through one interface, and the interface transmits both voice data and service data. |
Table 1 - Working Mode of a Voice VLAN
Aging Time of a Voice VLAN
In automatic mode, the device configured with voice VLAN automatically adds the interface that connects to a voice device to the voice VLAN after learning the source MAC address of the voice data sent from the voice device, and controls the number of the interfaces in the voice VLAN through the aging mechanism.
When the aging time expires, if the interface enabled with voice VLAN does not receive any voice data from the voice device, the interface that connects to the voice device will be deleted from the voice VLAN. If the interface enabled with voice VLAN receives voice dataagain from the voice device, the interface that connects to the voice device will be automatically added to the voice VLAN again.
In manual mode, the voice VLAN is not affected by the aging time
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