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[David Ames <david.ames@xxxxxxxxxxxxx>]

Public bug reported:

Setup:
Pike neutron 11.0.2-0ubuntu1.1~cloud0
OVS 2.8.0
Jumbo frames setttings per: https://docs.openstack.org/mitaka/networking-guide/config-mtu.html
global_physnet_mtu = 9000
path_mtu = 9000

Symptoms:
gre_sys MTU is 1472
Instances with MTUs > 1500 fail to communicate across GRE

Temporary Workaround:
ifconfig gre_sys MTU 9000
Note: When ovs rebuilds tunnels, such as on a restart, gre_sys MTU is set back to default 1472.

Note: downgrading from OVS 2.8.0 to 2.6.1 resolves the issue.

Previous behavior:
With Ocata or Pike and OVS 2.6.x
gre_sys MTU defaults to 65490
It remains at 65490 through restarts.

This may be related to some combination of the following changes in OVS which seem to imply MTUs must be set in the ovs database for tunnel interfaces and patches:
https://github.com/openvswitch/ovs/commit/8c319e8b73032e06c7dd1832b3b31f8a1189dcd1
https://github.com/openvswitch/ovs/commit/3a414a0a4f1901ba015ec80b917b9fb206f3c74f
https://github.com/openvswitch/ovs/blob/6355db7f447c8e83efbd4971cca9265f5e0c8531/datapath/vport-internal_dev.c#L186

** Affects: neutron
     Importance: Undecided
         Status: New


** Tags: serverstack uosci upgrade

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https://bugs.launchpad.net/bugs/1742505

Title:
  gre_sys set to default 1472 when using path_mtu > 1500 with ovs 2.8.x

Status in neutron:
  New

Bug description:
  Setup:
  Pike neutron 11.0.2-0ubuntu1.1~cloud0
  OVS 2.8.0
  Jumbo frames setttings per: https://docs.openstack.org/mitaka/networking-guide/config-mtu.html
  global_physnet_mtu = 9000
  path_mtu = 9000

  Symptoms:
  gre_sys MTU is 1472
  Instances with MTUs > 1500 fail to communicate across GRE

  Temporary Workaround:
  ifconfig gre_sys MTU 9000
  Note: When ovs rebuilds tunnels, such as on a restart, gre_sys MTU is set back to default 1472.

  Note: downgrading from OVS 2.8.0 to 2.6.1 resolves the issue.

  Previous behavior:
  With Ocata or Pike and OVS 2.6.x
  gre_sys MTU defaults to 65490
  It remains at 65490 through restarts.

  This may be related to some combination of the following changes in OVS which seem to imply MTUs must be set in the ovs database for tunnel interfaces and patches:
  https://github.com/openvswitch/ovs/commit/8c319e8b73032e06c7dd1832b3b31f8a1189dcd1
  https://github.com/openvswitch/ovs/commit/3a414a0a4f1901ba015ec80b917b9fb206f3c74f
  https://github.com/openvswitch/ovs/blob/6355db7f447c8e83efbd4971cca9265f5e0c8531/datapath/vport-internal_dev.c#L186

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