Comment 0 for bug 1399064

A linux bridge always adopts the smallest MTU of the enslaved devices. When no device are enslaved, it defaults to a MTU of 1500 and refuses to use a larger one. This is problematic when using bridges enslaving only virtual NICs (vnetX) like it's common with KVM guests.

Steps to reproduce the problem

1) sudo ip link add br-test0 type bridge # create an empty bridge
2) sudo ip link add br-test0 mtu 9000 # attempt to set MTU > 1500
3) ip link show dev br-test0 # confirm MTU

Here, 2) returns "RTNETLINK answers: Invalid argument". One (cumbersome) way around this is:

4) sudo modprobe dummy
5) sudo ip link set dummy0 mtu 9000 master br-test0

Then the bridge's MTU can be changed from anywhere to 9000.

This bug is especially annoying for the virtualization case because the KVM's tap driver will by default adopt the bridge's MTU on startup making it impossible (without the workaround) to use a large MTU on the guest VMs.

Additional information:

$ lsb_release -rd
Description: Ubuntu 14.04.1 LTS
Release: 14.04
$ apt-cache policy linux-image-3.13.0-41-generic iproute2
linux-image-3.13.0-41-generic:
  Installed: 3.13.0-41.70
  Candidate: 3.13.0-41.70
  Version table:
 *** 3.13.0-41.70 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-proposed/main amd64 Packages
        100 /var/lib/dpkg/status
iproute2:
  Installed: 3.12.0-2
  Candidate: 3.12.0-2
  Version table:
 *** 3.12.0-2 0
        500 http://archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: linux-image-3.13.0-41-generic 3.13.0-41.70
ProcVersionSignature: Ubuntu 3.13.0-41.70-generic 3.13.11.11
Uname: Linux 3.13.0-41-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.6
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: simon 4594 F.... pulseaudio
CurrentDesktop: Unity
CurrentDmesg: dmesg: klogctl failed: Operation not permitted
Date: Wed Dec 3 23:02:25 2014
HibernationDevice: RESUME=UUID=8e7a3220-158f-413b-81b2-55b236bb1f3f
InstallationDate: Installed on 2014-01-26 (311 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Alpha amd64 (20140124)
MachineType: LENOVO 2516CTO
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-3.13.0-41-generic root=/dev/mapper/crypt-root ro quiet splash cryptopts=target=crypt,source=/dev/sda1,lvm=crypt-root possible_cpus=4 nmi_watchdog=0 vt.handoff=7
RelatedPackageVersions:
 linux-restricted-modules-3.13.0-41-generic N/A
 linux-backports-modules-3.13.0-41-generic N/A
 linux-firmware 1.127.10
RfKill:
 0: phy0: Wireless LAN
  Soft blocked: no
  Hard blocked: no
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 02/14/2013
dmi.bios.vendor: LENOVO
dmi.bios.version: 6IET85WW (1.45 )
dmi.board.name: 2516CTO
dmi.board.vendor: LENOVO
dmi.board.version: Not Available
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Not Available
dmi.modalias: dmi:bvnLENOVO:bvr6IET85WW(1.45):bd02/14/2013:svnLENOVO:pn2516CTO:pvrThinkPadT410:rvnLENOVO:rn2516CTO:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable:
dmi.product.name: 2516CTO
dmi.product.version: ThinkPad T410
dmi.sys.vendor: LENOVO