The gateway NCP assists in cross-network session initiation and termination by providing network address transformations from the pool of network alias addresses for each network. In addition, the gateway NCP assists in activating explicit and virtual routes for use by cross-network sessions. The gateway NCP is a part of all the networks it connects. The resources attached to the gateway NCP, however, are owned by VTAMs in only one of the attaching networks.

The gateway NCP is attached to other subareas in the various networks. It connects with SDLC links, token rings, or channel attachments.

Every gateway NCP has one native network. The native network is identified by the NETID operand on the BUILD definition statement in the NCP generation definition. The native network contains the VTAMs that are allowed to own (activate) the subordinate resources (the physical units, LUs, peripheral links, and cross-subarea links) of the gateway NCP. The gateway NCP resources can be owned by any or all of the VTAMs in the native network, whether they are gateway VTAMs or not.

Every gateway NCP has one or more nonnative networks. Each nonnative network is identified by a NETWORK definition statement in the NCP generation definition. A nonnative network is one that is attached to the gateway NCP for the purpose of interconnection. A VTAM® in a nonnative network can activate the gateway NCP but cannot activate any of the NCP subordinate resources.

At least one gateway NCP is required for network interconnection. The gateway NCP performs the address translation necessary for cross-network session traffic.

A pool of alias addresses is used for cross-network sessions. The coding of the GWNAU definition statement determines whether VTAM can activate SSCP-SSCP sessions in some configurations, because it defines representations of cross-network logical units and SSCPs within the gateway NCP.

An SSCP can neither send nor receive a request to establish a cross-network session with another SSCP unless it knows that SSCP subarea and element address. A nongateway SSCP cannot get the address from the gateway NCP; thus, the nongateway SSCP depends on the GWNAU definition statement corresponding to the other-network SSCP to supply the alias address. You can code this address on the SUBAREA and ELEMENT operands of the CDRM definition statement that defines the other-network SSCP to the nongateway host.

In a back-to-back gateway configuration, a gateway SSCP must be able to tell its gateway NCP what address represents the other-network SSCP in the second gateway NCP. The address in the GWNAU definition statement for the other-network SSCP in the second gateway NCP is the same as that in the ADJNETSA and ADJNETEL operands of the GWPATH definition statement following the CDRM definition statement that defines the other-network SSCP to the gateway host.

Sometimes a request arrives at a gateway NCP before VTAM has determined the address. In this case, the gateway NCP rejects the request. At the same time, the NCP signals the destination SSCP to set up the session. The NCP knows the address of the destination SSCP only if the SSCP name on the GWNAU definition statement for the alias address to which the ACTCDRM request is sent matches an SSCP name that the NCP received in an ACTPU request. This SSCP name sent in an ACTPU request from a gateway SSCP is determined by the SSCPNAME start option.

If you are dividing a network into multiple networks and some subarea nodes are not extended network addressing nodes, group those subarea nodes, if possible, into one or more unique networks. You might want to change MAXSUBA to meet a particular addressing requirement for a newly created network.

If a nonextended addressing node VTAM activates a gateway NCP, the NCP must define the MAXSUBA operand on the BUILD and NETWORK definition statements. A nonextended addressing NCP, functioning as a gateway, does not support sessions with resources in nodes having subareas greater than MAXSUBA. The nonextended addressing node gateway cannot activate an explicit route to a subarea greater than MAXSUBA in that network.