[Users] Adding a VLAN tagged network

Hi, I've got a development oVirt system with four machines, one NFS storage, one engine, and two oVirt Node hosting boxes. All the machines have an ethernet interface with an IP on the 'real' network/subnet that they live on, and there's a private network linking second ethernet ports on the nodes and the NFS box that all the storage traffic goes over. I can run guests with IP addresses on the same 'real' network as the hosts, but now I want to add a second VLAN tagged network, but using the same physical interfaces, i.e. so that untagged packets on eth7 go to the ovirtmgmt network (as they do now) and tagged ones go to a new oVirt logical network. I can create the logical network through the web interface, but when I try to assign it to a node's ethernet interface it configures the VLAN interface correctly and creates the new bridge for the logical network, but it also removes the ovirtmgmt network from the untagged version of the interface with predictably bad results (this has mostly been diagnosed via a shell on the local console). Is there any way to make this work, or do I need to isolate the ovirtmgmt network onto a physically separate port, rather than just by means of VLAN? Ewan