Cross-domain considerations

If you want to use VTAM® services to access a CICS® region on another MVS™ image, you must ensure that the required cross-domain services are defined to the VTAMs involved.

For example, to be able to use a VTAM APPC connection between a CICS region (applid CICSHTH1) on MVS image MVSH and a CICS region (applid CICSHAJ1) on MVS image MVSJ:

  1. Define the cross-domain services (CDRSC) for accessing CICSHAJ1 in a member of the SYS1.VTAMLST library (or your own user.VTAMLST library) for MVSH.
  2. Issue a VARY ACT command on MVSH to activate the CDRSC definition for accessing CICSHAJ1.
  3. Define the cross-domain services (CDRSC) for accessing CICSHTH1 in a member of the SYS1.VTAMLST library (or your own user.VTAMLST library) for MVSJ.
  4. Issue a VARY ACT command on MVSJ to activate the CDRSC definition for accessing CICSHTH1.

For example, you could:

  1. Create the following CDRSC definition in a member of the VTAMLST library on MVSH:
    CDIDHAJ1 VBUILD TYPE=CDRSC
    *********************************************
    * CDRSC for access to applid CICSHAJ1 on MVSJ
    *********************************************
    CICSHAJ1 CDRSC CDRM=IYAMCDRM   MVSJ
  2. Issue the following command on MVSH to activate the cross-domain services to CICSHAJ1 on MVSJ:
    /V NET,ACT,ID=CDIDHAJ1
  3. Create the following CDRSC definition in a member of the VTAMLST library on MVSJ:
    CDIDHTH1 VBUILD TYPE=CDRSC
    *********************************************
    * CDRSC for access to applid CICSHTH1 on MVSH
    *********************************************
    CICSHTH1 CDRSC CDRM=IYALCDRM   MVSH
  4. Issue the following command on MVSJ to activate the cross-domain services to CICSHTH1 on MVSH:
    /V NET,ACT,ID=CDIDHTH1

VTAM LU alias facility

Specifying a prefix string on the LUAPFX parameter of the CICS APPL statement indicates that VTAM is to generate LUALIAS names for dynamic cross-network CDRSCs in session with the CICS region defined by the APPL statement. This enables CICS to use an LU alias for autoinstalled terminals and work stations and thus ensure unique names in a CICSplex comprising terminal-owning and application-owning regions (TORs and AORs). VTAM generates the LUALIAS names dynamically.

CICS supports both forms of the VTAM alias function--predefined and dynamic--only where shown in the following table:

CICS-to-CICS APPC connections (APPL definitions) APPC devices (LU definitions) Terminals
Synclevel 1 Synclevel 2 Synclevel 1 Synclevel 2
Predefined alias only Predefined alias Dynamic alias Predefined alias Dynamic alias Predefined alias Dynamic alias
VTAM Yes Yes Yes Yes Yes Yes Yes Yes
CICS Yes No Yes Yes No No Yes Yes
Notes:
  1. The LU alias is used as the NETNAME for terminals and work stations that logon to a CICS region.
  2. CICS does not support LU alias for synclevel 2 connections (LUTYPE 6.1 and 6.2) and ignores any LU alias for these LU types, and continues to use the network name defined in the VTAM APPL statement.

Dynamic LU alias support

CICS supports the use of a dynamic LU alias for CICS terminals and workstations that are autoinstalled only. You enable dynamic LU alias support by specifying LUAPFX on the VTAM APPL definition for any CICS terminal-owning region that could receive duplicate netnames. Also, when starting VTAM, specify the following options on the VTAM START command:

VTAM generates a dynamic LU alias only if LUAPFX is specified on the CICS APPL statement and the resource comes from another network. That is, it has a different network name from the network to which the CICS region belongs.

When to use dynamic LU alias

Use dynamic LU alias where:

Predefined LU alias support

CICS supports the use of a predefined LU alias for CICS terminals and workstations that are explicitly defined and those that are autoinstalled. You can also use a predefined LU alias for CICS regions that communicate using CICS intersystem communication (ISC). You enable predefined alias support by specifying LUALIAS=alias on any cross-domain resource (CDRSC) that needs a specific alias.

Note:
A terminal or APPC sync level 1 work station that is defined to CICS on an explicit resource definition (that is, it is not autoinstalled) and is in a different network, requires a CDRSC definition with a specific alias on the LUALIAS parameter. This overrides the dynamic generation of an alias where LUAPFX is specified on the CICS region's APPL statement. To ensure that CICS can match the VTAM LU alias with the installed terminal definition, the LUALIAS value must match the NETNAME specified on the CICS TERMINAL resource definition.

An LUALIAS option in the CDRSC is effective if the resource comes from another VTAM domain (or network). That is, it is not used if the resource comes from the same MVS image, but is used if the resource comes from another MVS image regardless of whether it is from the same sysplex, another sysplex in the same network, or from a different sysplex. If an LU alias is predefined, a dynamic LU alias is not generated.

When to use predefined LU alias

Use predefined LU alias where:

Cross-network devices that need predefined LU alias

If the following VTAM cross-network resources are to be connected to a CICS region that is defined to VTAM with LUAPFX specified on its APPL statement, they must each have a CDRSC LUALIAS=netname entry:

Choosing an LUAPFX value

When choosing an LUAPFX value, consider the scope of this parameter within the CICSplex, and also consider its scope within the sysplex in which your CICS regions operate.

A predefined LUALIAS name is supplied to CICS for cross-domain and cross-network resources. All the CICS regions in an MVS image share the same VTAM and are in the same domain. A CICS region in a different MVS image uses a different VTAM and is thus in a different domain. Resources coming from one VTAM to another, but which share the name NETID, are cross-domain resources

A dynamic LUALIAS name is only supplied to CICS for cross-network resources. A resource is a cross-network resource if it has a different network id (NETID). VTAM ensures that all the dynamic LUALIAS names assigned in one MVS image are unique. However, CICS needs network names to be unique across MVS images so that we do not get network name clashes in AORs.

It is important that all CICS regions across all connected networks use unique APPLIDs. This is true whether or not dynamic LUALIASs are used--it just makes it more important.

To ensure that all VTAM resources in a CICSplex have unique network names, use the LUAPFX prefix as follows:

Other considerations when using LU aliases

The following are some other factors to consider when you are planning to use VTAM LU aliases with CDRSC resources:

Predictable termids
If you need autoinstalled terminal resources to have a predictable and reproducible TERMID for such things as temporary storage queue names and START requests, you may need to modify your autoinstall user-replaceable module (URM) to select a reproducible TERMID from the network qualified name (NQNAME) supplied in the CINIT or the BIND.

There is an example of such code (commented-out) in the sample autoinstall URM, which extracts the network qualified name from the CINIT and BIND. The example illustrates how to create a TERMID from the last non-blank character of the NETID and the last 3 non-blank characters of the real network name (NETNAME).

MVS workload management
If your MVS workload policies specify LU name classifications, remove the LU name for any cross-network resources that are autoinstalled by CICS.
Recovery and persistent sessions support
Resources for which CICS uses any VTAM LU alias (predefined or dynamic) and which come from a different network are not cataloged by a CICS region that is not using persistent session. This means the terminal sessions for the resources cannot be recovered during an emergency restart.

Resources for which CICS uses any VTAM LUALIAS (predefined or dynamic) and which come from a different network are catalogued if CICS is using persistent sessions. This enables CICS to restore resource terminal session information from the CICS catalog pending recovery of the session from VTAM. However, if the resource does not persist, the resource is deleted during an emergency restart.

This action is necessary because VTAM may have been restarted, which would cause dynamic LU aliases to be reissued to different sessions. CICS is unable to tell if VTAM has been restarted, and CICS cannot tell the difference between a predefined and a dynamic LU alias.

CLSDST PASS
If you ISSUE PASS (CLSDST PASS) for a terminal that uses a dynamic LU alias to pass control to another CICS region in another MVS image, the resource is known by a different network name in the receiving CICS. This is true if the APPL statement of only one or both the CICS regions specify LUAPFX to activate dynamic LU alias.
Generic resources
If a number of generic resource TORs are in two different MVS images, a terminal or work station that logs on to one image is assigned a different network name if it logs off and logs on to a TOR in another image.
FEPI
FEPI front end systems are not supported by VTAM LU alias.
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