Sysplex distributor uses the dynamic XCF interfaces (DYNAMICXCF parameter on the IPCONFIG or IPCONFIG6 statements) to distribute all incoming packets to target stacks. For information about how to determine route selection to tier 1 non-z/OS targets, see Sysplex distribution with DataPower.
Using dynamic XCF interfaces has several advantages, such as simplified configuration, because you can leverage the existing XCF communication links in your sysplex environment and do not have to define separate communication paths to all target systems. This is especially useful in scenarios where target systems do not have direct network connectivity, but rather rely on the network connectivity of the routing stacks.
Sysplex distributor also includes some optimization logic that enables it to select alternative network paths for forwarding DVIPA packets. This optimization logic is automatically performed by the sysplex distributor in the following configurations:
These optimizations provide the best performance characteristics for forwarding DVIPA packets within the same CPC.
For configurations where the routing and target stacks are in different CPCs, there are some scenarios where it might be desirable to use interfaces other than the dynamic XCF interfaces for forwarding distributed DVIPA packets:
In these configurations, forwarding DVIPA packets over these alternative network connections can actually improve performance (that is, reduce latency and CPU cost) while reducing the usage of sysplex XCF interfaces.
You can use the VIPAROUTE statement in the VIPADYNAMIC block in the TCP/IP profile to influence the sysplex distributor's logic in selecting a route and interface to forward DVIPA packets to a target stack. Using the VIPAROUTE statement, you can indicate which IP address on the target stack is to be used as the destination or target IP address during the route lookup selection. When sysplex distributor processes the incoming packet, it determines whether a matching VIPAROUTE statement has been defined, and if it has, the TCP/IP stack returns the best available route to reach the target IP address. The incoming packet is then encapsulated using a destination of the VIPAROUTE target IP address in the outer header, and forwarded to the target stack. Generic routing encapsulation (GRE) is used to encapsulate IPv4 packets, while an outer IPv6 header is used to encapsulate IPv6 packets. This enables you to select the optimal interface for forwarding DVIPA packets across different CPCs, while retaining the optimized communication paths when the routing and target stacks are on the same MVS image, the same CPC, or both.
If you do not want to use the dynamic XCF interfaces at all, you must define the dynamic routes so that the cost to reach the IP address on the target stack using the dynamic XCF interfaces is higher than the cost to reach that IP address using other interfaces. For more information, see Steps for configuring OSPF and RIP (IPv4 and IPv6). If this is not done, it is possible that dynamic XCF interfaces will be used if the normal routing tables select those interfaces. By ensuring that the route using the dynamic XCF interfaces has a higher cost, you can exploit the other interfaces for forwarding most DVIPA traffic, yet maintain the dynamic XCF interface as a backup should the preferred network interfaces fail.
In the following cases, even though VIPAROUTE was specified, the dynamic XCF interface is used for distribution:
When these conditions are detected, messages are issued at the distributing stack, as well as when the distributing stack first attempts to route a connection request to the target stack.
The dynamic XCF address must still be configured even if VIPAROUTE definitions are used, as sysplex distributor continues to use a dynamic XCF address to identify every target TCP/IP stack in the sysplex. In addition, there are several functions that continue to depend on dynamic XCF connectivity for intra-sysplex communications: