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[Elvira García Ruiz <1951559@xxxxxxxxxxxxxxxxxx>]

Public bug reported:

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.

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

** Affects: neutron
     Importance: Undecided
     Assignee: Elvira García Ruiz (elviragr)
         Status: New

** Changed in: neutron
     Assignee: (unassigned) => Elvira García Ruiz (elviragr)

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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:
  New

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.

  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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