This paper discusses a simple and speedy routing method in large-capacity optical Wavelength Division Multiplexing (WDM) networks. The large-capacity WDM network is necessary to accommodate increasing traffic load in future. In this large-capacity WDM network, each link has many fibers and a huge amount of optical data can be transmitted through these fibers simultaneously. Optical path is configured for transmitting optical data by wavelength reservation including routing and wavelength assignment (RWA). Since traditional RWA methods have to treat much information about available wavelengths in each fiber, it is difficult to resolve RWA problem on time. In other words, the electrical processing becomes the bottleneck in the large-capacity WDM network. Therefore, a simple and speedy RWA method is necessary for the large-capacity WDM network. In this paper, we propose the simple and effective RWA method which considers reduced information as Network Map. The objective is to improve the network performance by using multiple fibers effectively. The complex processing is not suitable for data transmission because the switching operation must be done in very short time for one request. In addition to this, it is not practical to collect detailed network information frequently. The proposed wavelength assignment method assigns wavelength more uniformly than traditional method, and therefore, the proposed routing method can select routes without considering detailed information about each wavelength state. The proposed routing method needs only local information and reduced network information. This paper shows that the proposed routing method can get suitable solution for large-capacity optical WDM networks through computer simulations. The proposed RWA method drastically improves the loss probability against other simple RWA methods. This paper also describes two types of optical switches with tunable or fixed wavelength conversions. The wavelength converters with relatively low technology becomes effective with the proposed RWA method in the large-capacity WDM network. This paper reveals that complex routing methods are not necessary for large-capacity optical WDM networks.