To enable this facility, code the LUGROUP operand on the PU definition statement for the PU, and code an LU group major node. The LUGROUP operand on the PU definition statement specifies the name of the model LU definition group that VTAM® uses when dynamically defining LUs. The LU group major node contains the model definition statements. Dynamic definitions for LUs are built using the model LU definitions contained in this major node.

After you enable this facility, VTAM uses the IBM-supplied selection of definitions for dependent LUs (SDDLU) exit routine to generate LU names for the dynamically built LU definition statements. The SDDLU exit routine controls the selection of the model LU and the LU name. If you prefer to use your own naming convention or method of selecting an LU model, you can write your own exit routine. For information about coding your own exit routine, see z/OS Communications Server: SNA Customization.

If you use the IBM-supplied SDDLU exit routine, code the LUSEED and LUGROUP operands on the PU definition statement. The LUSEED operand provides a pattern name that is used with the SDDLU exit routine to create a name for the dynamically created LU definition statements.

Figure 1 illustrates VTAM dynamically defining dependent LUs attached through a 3174 cluster controller, and the steps listed after the table explain the process.

Figure 1. Definition building for dynamically defined dependent LUs

1. The LU at local address 003 powers on. The 3174 sends VTAM a Reply/PSID NMVT indicating a 3278 model 005 device has powered on at address 003.

2. VTAM uses the Reply/PSID NMVT to do the following actions:

   1. Check the 3174 PU definition statement for a definition of an LU at address 003
   2. Determine if the 3174 supports dynamic definition of dependent LUs (LUGROUP operand is coded), if the 3174 PU definition statement has no definition of an LU at address 003.
3. VTAM matches the LU group name specified in the 3174 PU definition statement with an LU group name in the LUGROUP definition statement.
4. The default SDDLU exit routine uses the seven-character machine type and model number as the model LU name. VTAM serially checks the LU group for a model LU statement that matches the model name supplied by the SDDLU exit routine. In this example, VTAM finds the second definition statement (3278@). The characteristics specified in that statement are used.
5. To create the LU name, the default SDDLU exit routine uses the value on the LUSEED operand. In this example, the three # characters specified on the LUSEED operand are replaced with the numeric local address. Therefore, the name of the LU is LU003. For more options, see the z/OS Communications Server: SNA Resource Definition Reference.
6. VTAM builds a dynamic definition for the LU. The dynamic definition is rebuilt if that device powers off and a new device powers on at the same local address. Also, the dynamic definition is rebuilt if the device powers off and powers back on after its LU group major node has been deactivated and then reactivated.