Compatibility between nonextended and extended network addressing nodes

You can use the MAXSUBA start option or the MAXSUBA operand in NCP to enable subareas with different addressing structures to communicate. For a network containing nonextended addressing nodes, the MAXSUBA start option specifies the highest subarea value that can be used throughout the network. A MAXSUBA of 63, for example, defines a network with up to 63 subareas and 1024 elements in each subarea. Code the MAXSUBA start option in the start option list if you want VTAM® to communicate with nonextended addressing nodes, or if the MAXSUBA operand is coded in an NCP that VTAM will communicate with.

Extended network addressing is a network addressing structure that increases the size of the subarea address up to 255 in conjunction with up to 32 768 elements. VTAM extends host element addressing to 65 535 elements.

Nonextended addressing nodes can continue to use this structure. They can, however, participate in an extended network addressing network only when the following two conditions are met:

A compatibility program temporary fix (PTF) must be installed on all nonextended addressing nodes that communicate with extended network addressing nodes.
All nodes that communicate with nonextended addressing nodes must define a MAXSUBA. Extended network addressing nodes need this information to decode 16-bit network addresses. The value specified for MAXSUBA must be the same at all nodes.

In addition, the following restrictions apply when extended and nonextended addressing nodes coexist in the same network:

VTAMs that are nonextended addressing nodes cannot communicate with any logical unit whose element address exceeds the maximum element number defined by the subarea/element split.
VTAMs that are nonextended addressing nodes cannot communicate with any node whose subarea is greater than the MAXSUBA.

Figure 1 illustrates an element address incompatibility. Domain 1 contains nodes that use extended network addressing. Domain 2 contains nodes that use nonextended addressing node addresses. All nodes have defined a MAXSUBA of 63. Although all of the subareas are within the MAXSUBA, some of the element addresses exceed the maximum, which is 1023. Following are two address constraints on this network:

In Domain 1, the terminals T1 and T3 can communicate with any subarea because their element addresses (100) are less than 1024. T2 and T4, however, cannot communicate with host A40M because their element addresses (1500 and 2000) are greater than 1023.
In Domain 2, T5 and T6 can communicate with either host. However, they cannot communicate with any application program in host A60M whose element address is greater than 1023.

Figure 1. Element and subarea address incompatibility in multiple-domain environment

These limitations can be overcome through SNA network interconnection (SNI), which can be used to place nonextended addressing nodes and extended network addressing nodes into separate networks. See Connecting multiple subarea networks for information about implementing SNI.

Although it is not recommended, a network containing nonextended addressing nodes can have subareas whose subarea number is greater than the MAXSUBA. The nonextended addressing nodes are unable to communicate with these subareas. In this situation, the following constraints apply:

A nonextended addressing node VTAM cannot appear in any route whose endpoints have a subarea address greater than MAXSUBA.
A nonextended addressing node VTAM cannot be adjacent to a node whose subarea is greater than MAXSUBA.
A nonextended addressing node NCP can be adjacent to a node whose subarea is greater than MAXSUBA, but only if the NCP is used as an intermediate routing node (IRN). This requires a PTF.

Table 1 and Table 2 show the adjacent and endpoint node subarea requirements.

Table 1. Adjacent node subarea requirements for multiple-domain environment
	Pre-ENA VTAM	ENA VTAM subarea ≤ MAXSUBA	ENA VTAM subarea > MAXSUBA
Pre-ENA NCP	Yes	Yes	Yes³
ENA VTAM Subarea < MAXSUBA	Yes	Yes	Yes
ENA VTAM Subarea > MAXSUBA	No	Yes	Yes
Notes: Yes: These two nodes can be endpoints. No: These two nodes cannot be endpoints. Pre-ENA is used as an IRN only.

Table 2. Endpoint node subarea requirements for multiple-domain environment
	Pre-ENA VTAM	ENA VTAM subarea ≤ MAXSUBA	ENA VTAM subarea > MAXSUBA
Pre-ENA NCP	Yes	Yes	No
ENA VTAM Subarea < MAXSUBA	Yes	Yes	Yes
ENA VTAM Subarea > MAXSUBA	No	Yes	Yes
Notes: Yes: These two nodes can be endpoints. No: These two nodes cannot be endpoints.

Figure 1 illustrates these constraints. The third domain contains extended network addressing nodes whose subarea addresses are greater than MAXSUBA.

The following communication restrictions apply to the network in Figure 1:

None of the Domain 2 terminals can have a session with host A250M. Conversely, the Domain 3 terminals cannot have a session with host A40M.
All of the terminals in Domain 3 can communicate with any application program in Domain 1, and all of the terminals in Domain 1 can communicate with any application program in Domain 3.
NCP A30NV2 is a nonextended addressing node, but can still act as an IRN between Domains 1 and 3. However, the NCP terminal, T6, cannot communicate with Domain 3.

As with the element address problem, subarea address incompatibilities can be avoided by using SNI to place nodes in separate networks. See Connecting multiple subarea networks for information about implementing SNI.