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This functional spec extends this design by allowing tagged interfaces to be utilized at the physical interface level:

The admin simply needs to create any 'vlan#' devices, and CloudStack uses them as physical devices.

CloudStack doesn't support defining actual physical devices to be used; instead it uses "traffic labels" to allow the admin to specify which bridges to use for what traffic, and then cloudstack determines the physical devices from those bridges. It then uses those physical devices to create subsequent tagged interfaces/bridges dynamically. Normally, if CloudStack finds that a bridge is on a tagged interface, it then looks up the parent of that interface. A small patch simply keeps CloudStack from looking up the parent if the device is a vlan# device, so that the vlan# dev is instead treated as a physical device and subsequent tagged networks are created there. 

This implementation was chosen because it leverages much of the existing CloudStack code. In other words, it's a simple hack. Alternative ways of implementing this feature would include adding the ability for CloudStack to track/configure individual physical devices (my impression is that avoiding this was a conscious decision), which would make the feature independent of the "vlan#" device. This could be as simple as having a 'force-phys-dev' list that we check against.

MTU

Actual MTU values and what is included in them vary slightly between manufacturers and operating systems, but in general it should be kept in mind that Q-in-Q requires extra space in order to contain the new vlan tag, so switches that provide physical connections between KVM hosts need to accomodate that. The table below provides an idea of what MTU values have been tested to work. Generally speaking, an admin should increase the MTU on the applicable switch hardware in order to be compatible with existing MTUs in CloudStack system VMs and guest instances.

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