Low-entry networking nodes, also referred to as T2.1 nodes, were introduced in the mid-1980s to address the requirement for peer networking. The low-entry networking node was the first stage of the APPN evolution.

The T2.1 node allows peer-to-peer connection and provides the physical and session-level connectivity required to support logical unit type 6.2 (LU 6.2). T2.1 nodes use protocols with reduced system definition requirements. For example, link station roles are negotiated as primary or secondary during the connection phase, instead of, as in the case of subarea networking, being predefined.

Low-entry networking nodes do not implement a control point. With low-entry networking, you must predefine every partner resource (if it does not reside on this same node) along with the first hop (link) toward that resource (this is because neither searching nor topology exchanges are supported by low-entry networking nodes). This predefinition requirement is the primary drawback of low-entry networking nodes.

Figure 1 illustrates a T2.1 skeleton network. The lines connecting the three nodes represent networking infrastructure like LAN, serial lines, or frame relay ATM. The T2.1 nodes are low-entry networking nodes without any APPN functionality.

Figure 1. T2.1 peer connectivity

In the T2.1 connection illustrated in Figure 1, LU-LU sessions can be established between nodes A and B and nodes B and C. Because node B does not include a control point or any APPN functions, it cannot route sessions, and therefore node A and node C cannot establish LU- LU sessions.

Prior to APPN, when only T2.1 nodes were implemented in SNA, transferring data from node A to node C required either a direct physical link between node A and node C or an application program in node B that relays messages between nodes A and C.