The topology database consists of a local topology database, unique to a node, and a network topology database, whose entries are replicated across all network nodes in the same topology subnetwork. The topology database stores and maintains the nodes and the links (transmission groups or TGs) in the networks and their characteristics.

A component called the topology database manager (TDM) creates and maintains the topology database.

An APPN network node provides route selection services to itself and to its client end nodes. It maintains a network topology database that has complete and current information about the topology subnetwork or NETID subnetwork in which it resides. This information includes all the network nodes in the subnetwork and their node characteristics, and all the links interconnecting these network nodes and their link characteristics, thus creating a connectivity "map" showing the arrangement of nodes and links.

A network node uses the network topology database to compute routes for sessions that originate at LUs in its domain (that is, in itself and in client end nodes). Each route that a network node computes is the current least-weighted route from the node containing the origin LU (primary logical unit, PLU) to the node containing the destination LU (secondary logical unit, SLU) for the requested class of service. To determine an appropriate path through the network, the route selection algorithm first assigns weights to transmission groups and nodes. These weights are scalar values for each node and transmission group, based on the relative significance of the characteristics for the requested class of service.

An end node maintains only a local topology database, while a network node maintains both a local topology database and the network topology database. The network topology describes the network nodes and transmission groups between network nodes (the backbone). All network nodes have an identical copy of this data (other than transient differences while new topology information is in transit).

The local topology database stores the adjacent end nodes and the links to them. Changes in the local topology database are not propagated to other nodes.

End nodes have a limited topology database that is only used to establish CP-CP sessions to a network node server (NNS), and to inform the NNS of its links to other end nodes and network nodes.