'Return MTU for the network where the given port belongs to.
If the network or port cannot be obtained, or if MTU is not defined,
returns None.'

def _get_port_mtu(self, context, port_id):

    core_plugin = directory.get_plugin()
    if (not n_utils.is_extension_supported(core_plugin, 'net-mtu')):
        return
    try:
        port = core_plugin.get_port(context, port_id)
        return core_plugin.get_network(context, port['network_id'])[api.MTU]
    except (n_exc.PortNotFound, n_exc.NetworkNotFound):
        return
