The network setup in dom0 is the default (I've just downloaded the source tarball and then built and installed it. The only change I've made is to enable 8021.q support in the xenU kernel). Then you are correct, in domU I use vconfig to create a vlan interface and ifconfig to give it an address and bring it up. On systems whose NICs don't use the tg3 driver, this seems to work fine. I can tcpdump on the bridge in dom0 and see the tagged traffic coming and going. Here is some info from dom0 on one of the working systems.I have 3 machines, T60 thinkpad with e1000 NIC Dell PowerEdge 2950 with bnx2 NIC HP DL360 G4P with tg3 NIC All running xen 3.03 / 2.6.16.29-xen I have a netbootable image with 2.6.16.29-xenU. This image when it boots creates a vlan eth0.4002, and then tries to communicate on that interface to a central server.Please can you describe the setup in more detail, including the bridge and NIC setup in dom0. It sounds like you're trying to pass a VLAN trunk into a guest and then use vconfig inside the guest, but I didn't think you could do this with the standard linux bridge as it didn't support VLAN trunking.