Multiple connections between a type 2.1 node and a subarea node

An independent LU can connect to the subarea network through only a type 2.1 PU. A type 2.1 PU can be connected to the subarea network through a type 4 or type 5 PU. The type 2.1 PU connections are called adjacent link stations. An independent LU can connect to multiple adjacent link stations. Thus, the independent LU can have multiple, simultaneous connections to the VTAM® host.

If one connection fails, only the sessions using that connection must be reestablished; the other sessions are not affected. The independent LU can then use an alternative adjacent link station for any subsequent sessions. You can restart any sessions terminated by the first adjacent link-station failure and connect the independent LU through another adjacent link station.

Some of the independent LU session paths might involve type 2.1 nodes connected to a VTAM other than the VTAM of the required session partner. For these sessions, the intermediate VTAM views the independent LU as a CDRSC. The CDRSC that represents the independent LU can also have other concurrent sessions that use PU type 2.1 nodes connected to the VTAM of the required session partner.

If an independent LU is connected to VTAM by only LEN links and two application programs try to contact it, VTAM provides the first available adjacent link station specified on the ALSLIST operand for both application programs. Both application programs would then have parallel sessions through the same adjacent link station. If you want the application programs to use different adjacent link stations, use the adjacent link station selection function of the session management exit routine. For information about coding this exit routine, see z/OS Communications Server: SNA Customization.

Figure 1 shows how an independent LU can connect to VTAM with multiple connections.

Figure 1. Independent LU with multiple connections to VTAM

Following are sample definition statements to define the configuration in Figure 1. Note that you can support independent LU as either destination logical units, originating LUs, or both:

For independent LUs as originating LUs, code DYNLU=YES in your start list. VTAM determines the adjacent link stations dynamically.
For independent LUs as destination LUs, code CDRSC minor nodes in a CDRSC major node. If the independent LUs are connected to VTAM by only LEN connections, include the ALSLIST operand.

Start list (ATCSTRxx)

If the independent LUs are originating LUs, code the start option for dynamic definition of independent LUs (DYNLU=YES).

NCP major node

Code the following (do not code LU definition statements):

⋮
         LUDRPOOL NUMILU=2        NUMBER OF INDEPENDENT LUS
LGROUP   GROUP DIAL=NO            LEASED LINE
LLINE    LINE
⋮
PU1      PU
⋮
SGROUP   GROUP DIAL=YES           SWITCHED LINE
SLINE    LINE
⋮
PU2      PU
⋮

Switched major node

Code a switched major node:

VBUILD   TYPE=SWNET
PUA      PU    ADDR=03,                    STATION ADDRESS
               CPNAME=A500N,               CONTROL POINT NAME (TYPE 2.1)
               DISCNT=YES,                 DISCONNECT FACILITY
               PUTYPE=2                    PU TYPE 2.1

Note: You can code the DYNLU operand on the PU statement rather than in the start list.

CDRSC major node

If the independent LUs are destination LUs, define your independent LUs as CDRSCs:

VBUILD   TYPE=CDRSC
ILU1     CDRSC ALSLIST=(PU1,PU2),          ADJACENT LINK STATION NAMES
               ALSREQ=YES                  USE ALS LIST ONLY
ILU2     CDRSC ALSLIST=(PU1,PU2),          ADJACENT LINK STATION NAMES
               ALSREQ=YES                  USE ALS LIST ONLY