Tested OK on Xenial with Pike UCA:

16: gre_sys@NONE: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 65000 qdisc pfifo_fast 
master ovs-system state UNKNOWN mode DEFAULT group default qlen 1000
    link/ether 56:5c:6e:19:f5:4b brd ff:ff:ff:ff:ff:ff

However testing on artful currently blocked pending resolution of bug 1743746 
which prevents an increase in the MTU of the gre devices.

** Tags removed: verification-pike-needed
** Tags added: verification-pike-done

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

Status in Ubuntu Cloud Archive:
  In Progress
Status in Ubuntu Cloud Archive pike series:
  Fix Committed
Status in Ubuntu Cloud Archive queens series:
  In Progress
Status in neutron:
Status in linux package in Ubuntu:
Status in openvswitch package in Ubuntu:
  In Progress
Status in linux source package in Artful:
Status in openvswitch source package in Artful:
  Fix Committed
Status in linux source package in Bionic:
Status in openvswitch source package in Bionic:
  In Progress

Bug description:
  OpenStack Clouds using GRE overlay tunnels with > 1500 MTU's will observe 
packet fragmentation/networking issues for traffic in overlay networks.

  [Test Case]
  Deploy OpenStack Pike (xenial + pike UCA or artful)
  Create tenant networks using GRE segmentation
  Boot instances
  Instance networking will be broken/slow

  gre_sys devices will be set to mtu=1472 on hypervisor hosts.

  [Regression Potential]
  Minimal; the fix to OVS works around an issue for GRE tunnel port setup via 
rtnetlink by performing a second request once the gre device is setup to set 
the MTU to a high value (65000).

  [Original Bug Report]
  Pike neutron 11.0.2-0ubuntu1.1~cloud0
  OVS 2.8.0
  Jumbo frames setttings per: 
  global_physnet_mtu = 9000
  path_mtu = 9000

  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 

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