For some reason MTU still seem to be an occasional issue with trunked connections.
In this case I have a trunk connected to an Enterprise Linux 5 server. I use standard vlan seperation in Linux, and have the vlan interface bridged with virtual interfaces for Xen.
If a guest has the MTU set to the default of 1500, networking breaks. If I configure the guest's MTU to 1496, everything is fine.
As I understand it, Intel Pro/1000 PT is VLAN aware and should let the VLAN tag through, on top of the 1500 byte network package.
What is the trick to make this work? Is there a driver setting or some special way to configure the VLANs?
Dom0 is using the e1000e driver.
Support for jumboframes is disabled on the switches.
Strangely enough, this is only a problem for guests. Dom0 has no MTU problems, despite having connectivity on an alias of a vlan bridge (eg. vlan10:1)