Hi, I use Debian 6.0 and Debian 7.0 production servers with it's pre-built xen packages. Some of my vlan-s and interfaces uses mtu=9000. If I use mtu=9000 with my domU-s too, then it after a while starts to hang up all network traffic on that vif. The xen-create-image script writes a hint into my /etc/network/interfaces: "post-up ethtool -K ethX tx off" But, if I switch off tx checksuming, then it reduce back the mtu to 1500: Last line of my dmesg: [168858.716229] vif vif-2: eth2: Reducing MTU because no SG offload So, scenario1: I start with MTU=9000, and it just hangs up after a while, Or scenrario2: I switch off tx/gso/etc. and it switch back to MTU=1500, and pmtu-discovery will fix nfs mounts from servers, which has mtu=9000... Does sy. have an debian based out-of-the-box working setup with mtu=9000? Regards, György