Compatibility between extended network and extended subarea addressing nodes

Whereas extended network addressing supports a maximum subarea address of 255, extended subarea addressing allows you to specify subarea addresses higher than 255. Subarea addresses can be as high as 65 535.

However, defining networks with large numbers of subareas can increase the storage requirements for VTAM® and NCP. To control the maximum number of subareas in a network, use the MXSUBNUM start option and the NCP operand SALIMIT to specify the maximum subarea size of the network. You can define VTAMs and NCPs in the network with varying values for the maximum subarea limit. However, the actual subarea addresses used must be within a common address range for the subarea nodes to communicate. Processors and communication controllers with levels of VTAM and NCP that do not support extended subarea addressing can exist in the same network with other processors and communication controllers that do. For subarea node communication to be supported, the actual subarea addresses used must be within 255 or the MAXSUBA value, whichever is smaller.

Migration can be simplified by installing levels of software that support extended addressing on all subareas in the network, with the maximum subarea limit specified to accommodate planned growth, before adding any subareas with a higher subarea address value. Failure to do so results in subareas that no longer have connectivity to other parts of the network.

The maximum subarea limit should, in general, be constant throughout the network. If the subarea limit cannot be constant, you can try different values with no loss of connectivity until a subarea address is used that is higher than the lowest subarea limit in the network. When moving to a lower subarea limit value, you should lower all subarea numbers before changing the subarea limit value.

The following subarea addressing restrictions apply to a multiple-domain network unless you use SNA network interconnection (SNI) to circumvent the addressing constraints:

Adjacent subareas can support different maximum subarea values and still communicate provided their actual subarea addresses fall within their common range. For example, a nonextended subarea addressing VTAM can communicate with an extended subarea addressing NCP that supports an SALIMIT value greater than 255 if the NCP subarea address is no greater than 255.
A subarea whose address is above the addressing supported by other adjacent subareas on a route cannot communicate to or through those subareas.
Subareas can communicate through other subareas with an address higher than they support provided there is an intermediate subarea with an address acceptable to both. For example, a nonextended subarea addressing VTAM could communicate through an NCP with a subarea address of 300. There would, however, have to be an intermediate VTAM or NCP supporting extended subarea addressing with an address no greater than 255 and an SALIMIT of at least 300.
An SSCP cannot activate any subareas with addresses greater than its addressing capabilities.
For an extended subarea addressing NCP to successfully send a CONTACT request unit to an adjacent extended subarea addressing NCP with an address above 255, the link station must be owned by an extended subarea addressing VTAM.

With extended subarea addressing, VTAM and NCP support the use of 16 explicit routes. You can define up to 16 explicit routes for each destination subarea. However, for an explicit route to have a number greater than 7, all subareas on the route must support extended subarea addressing, and all NCPs must have ERLIMIT=16 coded on the BUILD definition statement.