Cross-system Coupling Facility (XCF)

TCP/IP takes advantage of the communication capabilities of the XCF in a sysplex.

At the heart of the sysplex is the XCF. As its name suggests, XCF handles communication between logical partitions (LPARs) or CPCs. Communication between these LPARs is effectively (from a TCP/IP perspective) instantaneous.

Information such as workload, status, and data transmission occurs through the Coupling Facility. The information sharing is constant and continuous, allowing the independent z/OS images to know detailed information about the current status of all images within the sysplex.

TCP/IP takes advantage of the communication capabilities of the XCF in a sysplex in three different ways:

It maintains awareness of the status (health) of a TCP/IP instance within the sysplex.
It determines workload levels within each LPAR in the sysplex through Workload Manager (WLM).
It can send IP traffic among the LPARs.

In order for TCP/IP to be able to utilize the services of the Coupling Facility, VTAM must be configured to support XCF connections. This is normally accomplished by starting VTAM with XCFINIT=YES specified in ATCSTRxx.

As an implementation note, the Coupling Facility, if used, represents a single point of failure within a sysplex. Hence, an organization always implements a backup Coupling Facility which is ready in the event of a failure.