How does z/OS Communication Server use the Class of Service to choose a route?

z/OS® Communication Server chooses a route by comparing the actual characteristics of the available nodes and TGs to the allowed characteristic ranges specified in the requested COS. For each APPN COS entry, there are 1–12 LINEROW operands and 1–8 NODEROW operands. These operands give up to 12 acceptable sets of characteristics for the lines and up to 8 for the nodes in each COS. The WEIGHT parameter on the NODEROW and LINEROW operands is coded to indicate the desirability of that set of characteristics. The lower the value of the WEIGHT parameter, the higher the desirability of a node or TG that fits that set of characteristics.

Tip: The IBM-supplied COS definitions in COSAPPN are made up of 8-row LINEROW and NODEROW entries for all classes of service. The IBM-supplied COS definitions in ISTACST2 are made up of 12-row LINEROW entries for all classes of service except CPSVCMG and SNASVCMG, (which are made up of 8-row LINEROW entries), and 8-row NODEROW entries for all classes of service. The IBM-supplied COS definitions in ISTACST3 are made up of 12-row LINEROW entries and 8-row NODEROW entries for all classes of service. See Class of Service for more information.

For each of the LINEROW and NODEROW operands, an acceptable range is coded. For each potential line or node in a route, z/OS Communication Server compares the node or TG characteristics to the LINEROW or NODEROW operand values in the requested Class of Service. For an entire route to be acceptable for a given Class of Service, all nodes and TGs for that route must be acceptable for the requested Class of Service.

If more than one route fits the required characteristics of the Class of Service, z/OS Communication Server chooses the one with the least total weight, and therefore the most desirable characteristics. If no route matches the required characteristics of the Class of Service, the session fails (for example, if the Class of Service requires a secure route and VTAM® cannot find a complete route using only secure TGs).