A network node (NN) supports its own users and the end nodes it serves by providing directory and route selection services. It performs intermediate routing of data for sessions that cross it. The NN performs searches of the network to locate resources and calculates the best session route from the node of the primary LU to the node of the secondary LU, based on user-specified criteria.

Network nodes and their interconnections form an intermediate routing network. Network nodes use this intermediate routing network to perform searches for partner LUs, and to calculate the best available route for a session for their own resources and the resources of end nodes for which they are acting as network node servers.

Network nodes are distinguished from end nodes by the functions they provide. An NN provides network services for its own resources and the resources of the end nodes for which it acts as a network node server. For more information about network node servers, see Network node server.

To perform NN-provided network services, network nodes maintain a directory services database and a topology database. The directory services database contains information about resources in the network, including network node local resources, and is updated dynamically as a result of searches for partner LUs. The topology database includes information about network nodes and transmission groups (TGs) and is updated dynamically whenever there is a change to the intermediate routing network.

All directory requests from resources (such as LUs on served end nodes, or the network node resources) use the directory services of the NN. Therefore, the network nodes are able to collect and control directory information about the APPN network. Network nodes can originate and participate in APPN broadcast searches.