An independent LU can also have multiple sessions with a single LU (parallel sessions) or with several different LUs. The independent LU can assume the role of primary or secondary for any one or all of the sessions.

If one or more sessions are required between LUs for application transaction program communication, the independent logical unit establishes the necessary session connections on behalf of those application transaction programs.

The BSBUF buffer pool is used to provide common storage for boundary type 2.1, type 2, and type 1 peripheral-node-session control blocks. Depending on how you code the buffer pool start option, the BSBUF buffer pool can expand to support more sessions and contract as less session control blocks are needed. Because independent LUs can have multiple sessions (including parallel sessions), you should determine the average and maximum number of sessions associated with independent LUs that are directly attached to that VTAM® subarea node. You can then specify a value that prevents excessive buffer expansion and contraction.