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[Bug 1399064] Re: bridges cannot have a mtu > 1500 by themselves


Did this issue occur in a previous version of Ubuntu, or is this a new

Would it be possible for you to test the latest upstream kernel? Refer
to https://wiki.ubuntu.com/KernelMainlineBuilds . Please test the latest
v3.18 kernel[0].

If this bug is fixed in the mainline kernel, please add the following
tag 'kernel-fixed-upstream'.

If the mainline kernel does not fix this bug, please add the tag:

If you are unable to test the mainline kernel, for example it will not boot, please add the tag: 'kernel-unable-to-test-upstream'.
Once testing of the upstream kernel is complete, please mark this bug as "Confirmed".

Thanks in advance.

[0] http://kernel.ubuntu.com/~kernel-ppa/mainline/v3.18-rc7-vivid/

** Changed in: linux (Ubuntu)
   Importance: Undecided => Medium

** Tags added: kernel-da-key

** Changed in: linux (Ubuntu)
       Status: Confirmed => Incomplete

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  bridges cannot have a mtu > 1500 by themselves

Status in linux package in Ubuntu:

Bug description:
  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
    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
    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
  Uname: Linux 3.13.0-41-generic x86_64
  ApportVersion: 2.14.1-0ubuntu3.6
  Architecture: amd64
   /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
   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
   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

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