In an effort to facilitate recovery procedures, you might choose to define the same application programs or dependent LUs on more that one VTAM® simultaneously. Because SNA does not support duplicate resource definition, your search logic must be able to locate the active instance of the resource first. If a nonactive instance is located first, the session usually fails to set up. However, when an APPN broadcast search is performed, you have little or no control over which instance is located first, and this can result in intermittent session failures.

To allow continued use of duplicate resource definitions, the APPN search logic in VTAM was modified to recognize nonactive resource search replies. The search will continue in the APPN network for the active instance of the specified resource. If an active instance is located, the reply from that instance is used and session setup continues. If an active instance is not located, the reply from a nonactive instance is used instead.

Although this additional search logic allows the continued use of duplicate resource definitions in VTAM, it does this by increasing the amount of network search traffic generated by VTAM. If your application program or dependent LUs are not in the active state, you might incur this overhead, even if you are not using duplicate resource definitions. To minimize or avoid this additional searching, you can use DUPDEFS start option. This start option lets you specify what types of resources (application programs, dependent LUs, or both) have duplicate definitions on this and other VTAMs.

Specifying DUPDEFS=ALL means that application programs and dependent LUs may be duplicated on this and other VTAMs. DUPDEFS=APPL means that only application program definitions may be duplicated. DUPDEFS=DEPLU means that only dependent LU definitions may be duplicated. In all cases, if the target resource is one that might have duplicate definitions and the resource is not in the ACTIVE state, VTAM returns a reply indicating that searching should continue to find an active instance of the resource. If the target resource is one that does not have duplicate definitions, the search reply returned by VTAM will indicate that searching should not continue (regardless of the current state of the resource). DUPDEFS=NONE means that no duplicate definitions exist on this and other VTAMs, which prevents the additional search logic from being performed.