Networking on z/OS
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Cross-system Coupling Facility (XCF)

Networking on z/OS

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:
  1. It maintains awareness of the status (health) of a TCP/IP instance within the sysplex.
  2. It determines workload levels within each LPAR in the sysplex through Workload Manager (WLM).
  3. 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.

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