[OVN] Do not supply gateway_port if it's not bound to chassis

There is no reason to define gateway_port explicitly while create NAT
rules for FIPs in case where LRP is not bound to chassis. Doing so
will result in broken flows making NAT rules effectivelly ignored.

This is a case, when Logical_Router is pinned to chassis instead of LRP

Closes-Bug: #2083527
Change-Id: Iafaed72b6ac295db88497e16c54090952c410194

doc/source/admin/ovn/routing.rst

@@ -20,6 +20,21 @@ traffic, and also for FIPs.
    :alt: L3 North South non-distributed FIP
    :align: center
 
+When an external network connected to the router is represented by FLAT or
+VLAN network type, active chassis is identified by the external Logical Router
+Port. In practice this means, that LRP will have ``hosting-chassis`` property
+set in a ``status`` row for the external LRP. You can also check Chassis
+priorities for the LRP with ``lrp-get-gateway-chassis`` command. Changing the
+priority will result in traffic failover to another Chassis.
+
+In case of connecting another Geneve network to the router as external network
+(by creating ``access_as_external`` RBAC rule for such network), router itself
+will be pinned to Chassis rather than it's LRP. In this scenario Logical Router
+does have ``chassis`` property defined inside the ``options`` row.
+With that ``GATEWAY_PORT`` will not be defined for dnat_and_snat rules which
+are created for FIPs as this will make traffic to pass through the LRP that
+is not bound to any Chassis.
+
 
 Distributed Floating IP
 ~~~~~~~~~~~~~~~~~~~~~~~

neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_client.py

@@ -931,12 +931,17 @@ class OVNClient:
                    'external_ids': ext_ids}
 
         # If OVN supports gateway_port column for NAT rules set gateway port
-        # uuid to any floating IP without gw port reference - LP#2035281.
+        # uuid to floating IP without gw port reference - LP#2035281.
         if utils.is_nat_gateway_port_supported(self._nb_idl):
             router_db = self._l3_plugin.get_router(admin_context, router_id)
             gw_port_id = router_db.get('gw_port_id')
             lrp = self._nb_idl.get_lrouter_port(gw_port_id)
-            columns['gateway_port'] = lrp.uuid
+            # If LRP is not bound to a chassis, it means that router can be
+            # bound instead. In this case we do not want to define
+            # gateway_port LP#2083527.
+            if lrp.options.get(
+                    ovn_const.LRP_OPTIONS_RESIDE_REDIR_CH) == 'true':
+                columns['gateway_port'] = lrp.uuid
 
         if ovn_conf.is_ovn_distributed_floating_ip():
             if self._nb_idl.lsp_get_up(floatingip['port_id']).execute():

neutron/tests/unit/services/ovn_l3/test_plugin.py

@@ -1389,9 +1389,11 @@ class BaseTestOVNL3RouterPluginMixin():
             {'external_ip': '192.168.0.10', 'logical_ip': '10.0.0.0/24',
              'type': 'snat', 'uuid': 'uuid1'}]
         utils.is_nat_gateway_port_supported.return_value = is_gw_port
-
+        lrp_options = {}
+        if is_gw_port:
+            lrp_options[ovn_const.LRP_OPTIONS_RESIDE_REDIR_CH] = 'true'
         lrp = fake_resources.FakeOvsdbRow.create_one_ovsdb_row(
-            attrs={'options': {}})
+            attrs={'options': lrp_options})
         _nb_ovn.get_lrouter_port.return_value = lrp
         self.l3_inst.get_router.return_value = self.fake_router_with_ext_gw