A virtual route selection exit routine allows you to modify the ordered list of virtual routes (VRs) as specified in the class of service (CoS) entry. VTAM® then uses your new list of virtual routes to select a route for the session.

You can write a virtual route selection routine to be invoked before VTAM establishes any sessions between logical units and before VTAM is terminated (to enable it to clean up any necessary resources).

VTAM calls the virtual route selection exit routine when a session between a primary logical unit in the VTAM subarea and a logical unit in another subarea is about to be established. The virtual route selection exit routine is not called if both logical units reside in the same VTAM subarea. The exit is not called for independent PLUs. VTAM to reattach the subtask, thereby regaining use of the exit.

Only the list of routes within the local network is passed to the exit routine. The exit routine is scheduled only within the subarea of the primary logical unit.

The virtual route exit routine is not scheduled when:

• An SSCP session is established.
• An LU-LU session is established that is confined to the host's subarea. For example, two application programs communicating within the same host, or an application program communicating with a channel-attached terminal.
• A session setup is attempted, and no defined routes that are operational exist between the origin and destination subareas. In this case, the request to establish a session is rejected before the exit would have been scheduled.

You can use the DISPLAY EXIT command to display information regarding a virtual route selection exit routine. You also can use a MODIFY EXIT command to activate, deactivate, or replace the virtual route selection exit routine without interrupting VTAM processing. See Operator commands for VTAM exit routines for more information about using the MODIFY EXIT command to modify VTAM exit routines. See z/OS Communications Server: SNA Operation for more information about these commands.

The topics that follow contain information you need to write a virtual route selection exit routine.