VTAM® provides automatic, preferred route selection based upon dynamic network characteristics and requested COS. During session establishment, the network node server of the origin LU refers to the topology database to calculate and select the current best route through the APPN network from the primary LU to the secondary LU for the requested COS.

An NN calculates routes for sessions that originate at the LUs in it and at the ENs it serves. When a route is calculated, it is stored and can be reused. The stored routes are unique to the node and are never broadcast. The cache of stored routes is maintained automatically. When the maximum cache size (set by the NUMTREES start option) is exceeded, the least recently used routes are discarded first.

Route selection is based on how the actual characteristics of each node and TG along the possible paths match the characteristics required by the requested COS. The route that an NN selects is the current least-weight (or best) route from the node containing the origin LU to the node containing the destination LU.