Dear all: I start a VM with MTU size 1450, and login VM to check the mtu size, but this setting didn't work. The VM xml is like this: <interface type='bridge'> <mac address='52:54:00:54:14:f8'/> <source bridge='ovsbr1'/> <virtualport type='openvswitch'> <parameters interfaceid='a42e5b42-09db-4cfa-b198-d2ce62843378'/> </virtualport> <target dev='vnet0'/> <model type='virtio'/> <mtu size='1450'/> <alias name='net0'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/> </interface> However, the actual mtu size of VM is still 1500, as following: root@ubuntu-zhf:~# ip addr 2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether 52:54:00:54:14:f8 brd ff:ff:ff:ff:ff:ff The MTU size of both ovs bridge and vnet are 1450 in my hypervisor: root@ubuntu-191:~# ip addr show ovsbr1 16: ovsbr1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UNKNOWN group default qlen 1 link/ether 12:01:7b:5b:85:49 brd ff:ff:ff:ff:ff:ff inet6 fe80::1001:7bff:fe5b:8549/64 scope link valid_lft forever preferred_lft forever root@ubuntu-191:~# ip addr show vnet0 22: vnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc pfifo_fast master ovs-system state UNKNOWN group default qlen 1000 link/ether fe:54:00:54:14:f8 brd ff:ff:ff:ff:ff:ff inet6 fe80::fc54:ff:fe54:14f8/64 scope link valid_lft forever preferred_lft forever I use ovs to create vxlan networks and add VMs to the ovs bridge. Because of the limitations of vxlan, I need to set the mtu of vm to 1450. How to set the MTU of VM? Thanks. Below some information about my hypervisor: root@ubuntu-192:~# virsh -V Virsh command line tool of libvirt 3.4.0 See web site at http://libvirt.org/ Compiled with support for: Hypervisors: QEMU/KVM LXC UML OpenVZ VMware VirtualBox Test Networking: Remote Network Bridging Interface udev Nwfilter VirtualPort Storage: Dir Filesystem SCSI Multipath iSCSI LVM Miscellaneous: Daemon Nodedev SELinux Secrets Debug Modular root@ubuntu-192:~# qemu-x86_64 --version qemu-x86_64 version 2.9.0 Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers root@ubuntu-192:~# uname -a Linux ubuntu-192 4.4.0-62-generic #83-Ubuntu SMP Wed Jan 18 14:10:15 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux