``OVNL3RouterPlugin._port_update`` [1] is called AFTER_UPDATE the router port is created (for example, when a subnet is attached to a router). This event is guaranteed to be called after the Neutron DB has the resource (port) in the database. However, as the code highlights in the comment, this event can be called before the OVN NB database has the LRP resource created. The called method, ``update_router_port`` --> ``_update_router_port``, guarantees that the LRP update is executed only when the LRP exists but the LRP read [2] does not have this consideration.

This event should be replaced by an OVN DB event, checking the same conditions as in [1] and guaranteeing that the LRP resource is already created in the DB.

Example of this failure: https://zuul.opendev.org/t/openstack/build/3f7935d7ed53473898bbf213e85dfb61/log/controller/logs/dsvm-functional-logs/ovn_octavia_provider.tests.functional.test_driver.TestOvnOctaviaProviderDriver.test_create_lb_custom_network/testrun.txt

[1]https://opendev.org/openstack/neutron/src/commit/e8468a6dd647fd62eac429417c7f382e8859b574/neutron/services/ovn_l3/plugin.py#L372-L381
[2]https://opendev.org/openstack/neutron/src/commit/e8468a6dd647fd62eac429417c7f382e8859b574/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_client.py#L1809-L1811

U/S launchpad bug: https://bugs.launchpad.net/neutron/+bug/2056558