The dynamic definition of dependent LUs function defines LUs to a VTAM® network when the IBM® 3174 control unit containing the LUs powers on, rather than during major node activation. To enable the function, define the device to support dynamic definition. See z/OS Communications Server: SNA Resource Definition Reference and z/OS Communications Server: SNA Network Implementation Guide for more information about how to enable and use the function. With this support, VTAM builds LU definitions from reusable model LU definitions rather than using predefined LUs. The LU definitions are replaced or changed each time the device containing the LUs powers on. You can use this function to add, change, or relocate dependent LUs from a VTAM network without reactivating the major node.

The SDDLU exit routine allows you to specify the name of an LU being dynamically defined. The exit also allows you to specify which model LU VTAM uses to create the LU's definition. IBM supplies a default SDDLU exit routine in the VTAMUSER LOADLIB under the VTAM module, ISTEXCSD. The source for this default exit routine is stored in SYS1.SAMPLIB.

See Installing VTAM exit routines for more information. You can print a complete copy of the source code for the default exit from the library. VTAM uses the default exit unless you supply your own exit routine.

To enable the dynamic definition function, code the LUGROUP operand on the definition statement for the PU type 2 or type 2.1 device that supports the function. If you use the IBM supplied default exit, the LUSEED operand must also be coded on the PU's definition statement. See z/OS Communications Server: SNA Resource Definition Reference and z/OS Communications Server: SNA Network Implementation Guide for more information about these operands.

VTAM either initializes the SDDLU exit during VTAM initialization, or activates the routine after you issue a MODIFY EXIT,ID=exitname,OPTION=ACT command.

You can use the DISPLAY EXIT command to display information regarding an SDDLU exit. You also can use a MODIFY EXIT command to activate, deactivate, or replace the SDDLU exit routine without interrupting VTAM processing. See Operator commands for VTAM exit routines for more information about using the MODIFY EXIT command to modify VTAM exit routines. See z/OS Communications Server: SNA Operation for more information about these commands.