Gateway path selection function

The gateway path selection function can be used when a SNA network interconnection environment uses multiple gateway NCPs. The gateway VTAM® always attempts to establish a cross-network session by using the same gateway NCP over which the session already is established between two CDRMs. You can use the session management exit routine to modify this default gateway path selection. The gateway path selection function can either reorder the list of available gateway NCPs or delete entries from the list.

Each entry in the list is associated with a GWPATH definition statement following a CDRM definition statement. For example, assume the following CDRM and GWPATH definition statements are coded in the gateway VTAM (G10VTAM) in NET2:

VTAM01   CDRM
GWNCP30  GWPATH SUBAREA=30,ADJNET=NET1,ADJNETSA=1,ADJNETEL=1
GWNCP40  GWPATH SUBAREA=40,ADJNET=NET1,ADJNETSA=1,ADJNETEL=1

Assuming that both gateway NCPs (GWNCP30 and GWNCP40) are operative and all of the links are active, the first gateway NCP establishes the SSCP-SSCP session between the gateway VTAM (G10VTAM) and the VTAM host (VTAM01) in NET1.

The list of GWPATH definition statements passed to the session management exit routine is in the order in which they are coded. The gateway NCP that establishes the SSCP-SSCP session, however, is always placed first in the list. Therefore, in the example above, if GWNCP30 was not initially available for the SSCP-SSCP session, but was operative later, the order of the GWPATH entries would be reversed in the list.

Even though the gateway NCP is operative, links between the gateway NCP and the adjacent subarea might not be operative. As a result, a specific gateway NCP is selected to set up an LU session; however, the route from gateway NCP to the destination subarea is not available.

If the VTAM in NET1 (VTAM01) is a gateway SSCP that has multiple paths into network NET2, the gateway path selection function is called at session initiation to determine which gateway NCP should be used for the session. When called, the gateway path selection exit in VTAM01 is passed a list of paths arranged in the same order as the GWPATH macros in the cross-net CDRM definition, except that the NCP used for the CDRM-CDRM session is first in the list. The gateway path selection exit reorders the list and passes the results back to VTAM01. VTAM01 selects the first usable NCP from the reordered list for the session.

If the SSCP on the other side of the gateway (G10VTAM) can also function as a gateway SSCP into NET1, it will pass a single entry list to its gateway path selection exit. The single entry identifies the gateway NCP selected by VTAM01. Information about the gateway NCP selected by VTAM01 is delivered to G10VTAM on the CDINIT.

If VTAM01 and G10VTAM are in a back-to-back configuration separated by a null net, gateway path selection will be driven in both SSCPs. However, the CDINIT from the OLU network will not contain any information about the gateway NCP selected in the OLU network. The gateway path selection exit is driven in both VTAM01 and G10VTAM with the full list of gateway NCPs into the null network.