[libvirt-users] How to set the MTU size of VM interface

[netsurfed]

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:
***@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:
***@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
***@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:

***@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
***@ubuntu-192:~# qemu-x86_64 --version
qemu-x86_64 version 2.9.0
Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers

***@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

[Gianluca Cecchi]

Post by netsurfed
I start a VM with MTU size 1450, and login VM to check the mtu size, but
this setting didn't work.

[snip]
[snip]

Post by netsurfed
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.

You have to set it inside guest OS. So I presume still in
/etc/network/interfaces file for Ubuntu (see also "man interfaces")
See also this thread opened by me some days ago and Dominik's answer
related to new features and OVN internal dhcp server (if relevant in your
scenario)
https://lists.ovirt.org/archives/list/***@ovirt.org/thread/PDIZL2GL343GFWV5L6KA4KCPARJ4HWCE/

HIH,
Gianluca

[Laine Stump]

Post by netsurfed
I start a VM with MTU size 1450, and login VM to check the mtu size,
but this setting didn't work. 
[snip]
 
[snip]
 
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.
You have to set it inside guest OS.

Not necessarily. As far back as libvirt 3.1.0, if the qemu on the host
is new enough to support the "host_mtu" commandline option for network
devices, the MTU setting from libvirt's XML will be visible in the
device capabilities info of the device in the guest, and if the
virtio-net driver in the guest is new enough, it will automatically set
the MTU of the interface from the device capabilities info.

Post by netsurfed
So I presume still in
/etc/network/interfaces file for Ubuntu (see also "man interfaces")

Yes, if the host's qemu or the guest's virtio-net driver are not recent
enough, you'll need to set it in both the guest interface config and in
the libvirt XML config on the host.

Post by netsurfed
See also this thread opened by me some days ago and Dominik's answer
related to new features and OVN internal dhcp server (if relevant in
your scenario) 
HIH,
Gianluca
 
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