Peripheral subnetwork boundaries

A peripheral subnetwork boundary is a connection over a subnetwork boundary between a border node (VTAM® with APPN multiple network connectivity support enabled) and a network node with no border node function. The VTAM border node portrays an end node image to the network node, thus preventing topology information from being sent by the network node to VTAM.

VTAM offers the peripheral subnetwork boundary to provide APPN subnetwork interconnection with nodes that do not have border node function. Because the nonborder node in a peripheral boundary is not aware that the subnetwork boundary exists, compared to an extended boundary, there is considerable loss of control over search order, redundant searching, and looping search prevention. For these reasons, it is recommended that you use an extended boundary whenever possible. When using a peripheral boundary, it is wise to use a single subnetwork boundary connection to the peripheral subnetwork (the subnetwork of the nonborder node) to prevent locate requests from exiting the subnetwork over one boundary and then reentering over another boundary. If a peripheral subnetwork has more than one boundary, the user should customize routing by a user exit or routing definitions to ensure that the requests do not reenter the same peripheral subnetwork. This precaution is necessary to avoid possible failed sessions because of a looping locate path.

Guideline: Because these controls are not present for peripheral subnet boundaries, to avoid looping problems, limit connections to a single connection between networks when peripheral subnet boundaries are used.

Notes:

A peripheral subnetwork boundary can be used as either the first or last boundary (or both) on a multiple subnetwork search path. If there is an intermediate subnetwork on a multiple subnetwork search path (that is, neither the search originator nor the search target are members of that subnetwork), only extended border nodes can be used to manage the intermediate subnetwork's boundaries.
Not all network nodes are capable of attaching to a VTAM border node through a peripheral subnetwork boundary. Ensure the product and release support interoperability with peripheral border nodes (APPN option set 1013) before attempting to establish subnetwork boundaries between VTAM and a network node.
It is possible, although strongly discouraged, to get a peripheral boundary between two VTAMs that both support extended border node by setting BN=NO in one of the two nodes. This is strongly discouraged because it bypasses the valuable looping and redundant search controls that are present if the connection is an extended subnetwork boundary.