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["Dr. Jens Rosenboom" <j.rosenboom@xxxxxxxx>]

Public bug reported:

When booting an instance on a network with encapsulation type vxlan (and
thus automatically set MTU to 1450), this will also lower the MTU of the
integration bridge to that value:

$ ip link show br-int
6: br-int: <BROADCAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UNKNOWN mode DEFAULT group default 
    link/ether 7e:39:32:87:27:44 brd ff:ff:ff:ff:ff:ff

In turn, all newly created ports for routers, DHCP agents and the like,
will also be created with MTU 1450, as they are forked off br-int.

However, this will be incorrect if there are other network types in use
at the same time. So if I boot an instance on a network with type vlan,
the instance interface will still have MTU 1500, but the interface for
its router will have MTU 1450, leading to possible errors.

** Affects: neutron
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1590397

Title:
  MTU setting too low when mixing Vlan and Vxlan

Status in neutron:
  New

Bug description:
  When booting an instance on a network with encapsulation type vxlan
  (and thus automatically set MTU to 1450), this will also lower the MTU
  of the integration bridge to that value:

  $ ip link show br-int
  6: br-int: <BROADCAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UNKNOWN mode DEFAULT group default 
      link/ether 7e:39:32:87:27:44 brd ff:ff:ff:ff:ff:ff

  In turn, all newly created ports for routers, DHCP agents and the
  like, will also be created with MTU 1450, as they are forked off br-
  int.

  However, this will be incorrect if there are other network types in
  use at the same time. So if I boot an instance on a network with type
  vlan, the instance interface will still have MTU 1500, but the
  interface for its router will have MTU 1450, leading to possible
  errors.

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