Why VLANs Do Not Provide Effective QoS for VoIP
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We see constantly people using VLANs in order to add QoS for VoIP traffic. This should be somewhat obviously wrong, yet the myth that this is practical persists. VLANs are not a QoS vehicle but it is easy to set a QoS priority on a VLAN which makes it seam reasonable to use in this way. So we should look at why this myth does not hold up with real world VoIP traffic.
First, VLAN QoS priority is for the entire VLAN, not specific traffic on the VLAN. That should be obvious, but seems to be regularly missed.
Second, in nearly all VoIP scenarios, audio traffic is carried by RTP, the Real Time Protocol. So what we need is to get RTP traffic in and out of our network as quickly as possible.
Third, QoS only applies when a network is saturated. Digital networks do not have traffic issues until they hit 100% (at any given moment.) So only traffic in a queue has a prioritization potential.
Fourth, nearly all QoS needs are at the LAN to WAN barrier, not inside of the LAN. QoS on the LAN is still good, but often gets the focus of attention because it is "easy" and QoS going to the WAN is ignored because it is "hard", even thought that is where it really matters. So VLAN and QoS on the LAN are often sold as a "checkbox" trick to sucker customers into being able to say that they have QoS, even though not the QoS they were supposed to have. It's a sales gimmick. Also, QoS on the LAN makes it easy to sell new equipment, but QoS on the WAN is normally free, so there is no money to be made selling the more valuable solution.
How in a corporate network, a dedicated Voice VLAN will have to hit a router before leaving the network. Typically at this point, the VLANs are terminated and the resulting packets are comingled going onto the WAN. In most cases, the QoS is accidentally terminated before the WAN link is picked up and the QoS that we paid for and worked for never exists on the actual network. But this is not always the case.
What is the case is that the data on the Voice VLAN is not all RTP. Other traffic such as SIP, IAX2, HTTP, SSL, TFTP, FTP, DNS, patching, video streams and more are all on that LAN, even if no general traffic is. If the VLAN is so saturated as to be needing QoS, this traffic becomes a factor. When we are not saturated, this traffic is trivial (and QoS is pointless.) So in any situation where QoS is being used, this traffic matters.
The problem with a voice VLAN is that it doesn't prioritize any of that traffic, it is just an open LAN with the VoIP traffic being treated the same as any other traffic. So no QoS. Yes, if all other things are correct there is QoS of this VLAN over other VLANs, but that is only part of the QoS picture.
This is why, when QoS is needed (which is a different discussion) it is recommended that VLANs not be used for QoS but rather RTP itself be given end to end priority over other protocols so that matter where on the network the VoIP traffic is the voice traffic goes first (where there is congestion) and this will not only prioritize the real VoIP traffic, but will also allow it to maintain that priority past the point of VLAN comingling, should that point exist. And in some extreme cases, can even be carried onto the WAN link itself, which VLAN prioritization cannot.
So the use of VLANs for QoS is not just ineffective for VoIP, it actively undermines proper QoS that we would otherwise employ.
VLAN QoS is useful when we have a department, like the executives, that as a group need priority no matter what they are doing regardless of workload or protocol.
