A number of vendors recommend separating VOIP traffic from regular data traffic by putting all VOIP devices in...
a dedicated "voice VLAN". This approach has a number of advantages and also disadvantages. Here are some things to consider when deciding to use voice VLANs.
One advantage of using voice VLANs is that your VOIP devices do not have to contend directly with all the broadcasts and other traffic from data VLANs. This is especially effective if you... can afford dedicated hardware for the voice devices, but usually, multiple VLANs will be on a given switch, and trunked on the uplinks with 802.1Q, so this isn't a replacement for QoS.
Another advantage is that in some environments, using a voice VLAN can simplify network configuration. Specifically, marking packets for QoS is easier, because you don't have to specify the various ranges of TCP and UDP ports. All packets from the subnet get priority. Similarly, TFTP and the special DHCP services for IP Telephony are easier to configure when all IP Telephony devices are in a subnet.
On the other hand, this approach can be quite restrictive. For instance, if next year, you decide to implement soft phones on PCs in your normal data VLANs, your QoS configuration suddenly becomes much more complex. These VLANs also prevent you from taking advantage of most IP phones, which have a switch port that allows you to use your phone and PC on the same cable and switch port. Using a voice VLAN requires twice as many switch ports, which costs twice as much. And another disadvantage that absolutely cannot be overlooked is the administration cost associated with port-based VLANs. Fortunately, some of the vendors have proprietary ways of automating this adminstration.
One of the biggest factors in this decision is whether your network core is layer 2 or 3. If your VLANs are trunked all over your campus, then the voice VLAN can be pretty attractive because your voice end-points can potentially be on the same subnet as gateways, proxies, and other voice infrastructure. On the other hand, if your VLANs are small, and you have deployed layer-3 switching in the core and distribution layers, voice VLANs may not be worth the effort.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.