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[Alejandro Garcia <2101087@xxxxxxxxxxxxxxxxxx>]

Public bug reported:

Hi all. We have seen this behavior when using OVS ML2 plugin in a Kolla-
based deployed OpenStack. Basically, when changing the MTU of an
existing network, the change is not applied to already-bound Open
vSwitch ports.

The steps to reproduce are pretty straightforward:

1. Create an OVS network with MTU 1300.
2. Create a subnet, and deploy a virtual machine on it.
3. Change the network's MTU to 1400. If you then check the MTU for the previous OVS port (i.e. ovs-vsctl list interface) it will still be 1300.

This only affects existing ports. Newly created ports will use the newer
MTU. Workaround is to manually change the MTU (i.e. ovs-vsctl set int
<interface> mtu_request=1400) or to detach and re-attach the ports.
Looking at the ML2 plugin.py code (at
https://opendev.org/openstack/neutron/src/branch/master/neutron/plugins/ml2/plugin.py#L1305
) it appears that each agent should update the MTU for their ports.

** Affects: neutron
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2101087

Title:
  [ovs] MTU change is not applied to existing openvswitch ports

Status in neutron:
  New

Bug description:
  Hi all. We have seen this behavior when using OVS ML2 plugin in a
  Kolla-based deployed OpenStack. Basically, when changing the MTU of an
  existing network, the change is not applied to already-bound Open
  vSwitch ports.

  The steps to reproduce are pretty straightforward:

  1. Create an OVS network with MTU 1300.
  2. Create a subnet, and deploy a virtual machine on it.
  3. Change the network's MTU to 1400. If you then check the MTU for the previous OVS port (i.e. ovs-vsctl list interface) it will still be 1300.

  This only affects existing ports. Newly created ports will use the
  newer MTU. Workaround is to manually change the MTU (i.e. ovs-vsctl
  set int <interface> mtu_request=1400) or to detach and re-attach the
  ports. Looking at the ML2 plugin.py code (at
  https://opendev.org/openstack/neutron/src/branch/master/neutron/plugins/ml2/plugin.py#L1305
  ) it appears that each agent should update the MTU for their ports.

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