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Reviewed:  https://review.opendev.org/c/openstack/neutron/+/818443
Committed: https://opendev.org/openstack/neutron/commit/0725533a6fa6a42d361e518b027f69da8e1e1ec5
Submitter: "Zuul (22348)"
Branch:    master

commit 0725533a6fa6a42d361e518b027f69da8e1e1ec5
Author: Elvira García <egarciar@xxxxxxxxxx>
Date:   Thu Nov 18 18:08:13 2021 +0100

    [OVN] Fix gateway_mtu option should not always be set
    
    OVN Driver currently fixes gateway_mtu MTU to the provider MTU value
    without considering if the private networks in the associated router
    have greater MTU values than the provider. This is unnecesary and
    adds extra actions for each packet. This patch fixes that, as now
    gateway_mtu is only set in case the provider MTU is smaller than the
    private MTU.
    
    The changes in create_router_port and delete_router_port were necessary
    as there could be a use case when the user first sets the gateway
    router and later adds subnets from networks with greater MTU, so this
    parameter needs to be checked after adding a subnet.
    
    Closes-Bug: #1951559
    
    Signed-off-by: Elvira García <egarciar@xxxxxxxxxx>
    Change-Id: If56f1a3dcdc8c57303d5641df79ea919ba7c170d


** Changed in: neutron
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1951559

Title:
  [OVN] Router ports gateway_mtu option should not always be set

Status in neutron:
  Fix Released

Bug description:
  High level description:
  If a neutron router is connected to a provider network 'A' and private geneve networks.  If the mtu of private networks (1442 in normal cases) is lesser than that of provider network (1500), then there is no need for neutron ml2ovn to set options:gateway_mtu=1500 even if the config option to enable gateway mtu is set.

  Impact:
  This is important because the gateway_mtu option makes OVS use the check_pkt_larger() function, which is not necessary in most cases and is currently not hardware-offloadable. This means the traffic from VMs to external networks is currently not offloadable because of it.

  Step-by-step reproduction steps:
  1. Modify ml2_conf.ini: Add ovn_emit_need_to_frag = True in [ovn]

  2. Wire up the networks in a router:

  $ openstack network create net1
  $ openstack subnet create --subnet-range 192.168.100.0/24 --network net1 subnet1
  $ openstack router create r1
  $ openstack router add subnet r1 subnet1
  $ openstack router set --external-gateway public r1

  3. Check MTUs for each network:
  By default, net1 will be 1442 and private will be 1500, so mtu_gateway shouldn't be set

  4. Check if gateway_mtu was set on the Logical_Router_Port associated to the gateway
  $ ovn-nbctl list Logical_Router_port | less
  In this case, it shouldn't be set.

  Expected results:
  gateway_mtu is set in the Gateway LRP options for r1 only if provider MTU < private MTU.

  Actual results:
  gateway_mtu is always set if ovn_emit_need_to_frag is enabled.

  more info at [0]

  [0] https://bugzilla.redhat.com/show_bug.cgi?id=2019938

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