Primary and secondary routing

The following information applies to IPv4 datagrams. For primary and secondary router specifications for IPv6 datagrams, see the INTERFACE statement contained in the z/OS Communications Server: IP Configuration Reference.

If only one TCP/IP instance is using the OSA-Express, or when multiple TCP/IP instances are using the same OSA-Express device, and you wish all instances to share the same physical MAC address of the device, you can optionally have the OSA route packets to unregistered IP addresses by designating a TCP/IP instance as the primary router or secondary router. For a given OSA-Express, only one TCP/IP instance can be registered as the primary router and one or multiple TCP/IP instances can be registered as the secondary router, depending on the level of the OSA-Express. (See the description of the PRIROUTER and SECROUTER parameters on the INTERFACE-IPAQENET OSA-Express QDIO interfaces in z/OS Communications Server: IP Configuration Reference for details on how to designate a primary or secondary router.) If you define a TCP/IP instance as a primary or secondary router, you must also enable IP forwarding (using IPCONFIG DATAGRAMFWD in the TCP/IP profile).

When the device receives a datagram, OSA-Express routes the datagram as follows:

If the datagram is destined for a registered IP address, OSA-Express routes the datagram to the TCP/IP instance that registered the IP address.
If the datagram is destined for an unregistered IP address, OSA-Express routes the datagram to the TCP/IP instance that is registered as the primary router. If no TCP/IP instance is registered as the primary router, OSA-Express routes the datagram to one of the TCP/IP instances that is registered as the secondary router. If no TCP/IP instance is registered as a secondary router, OSA-Express discards the datagram.

Note: If a device becomes inactive for any reason, then OSA-Express unregisters any IP addresses that had been registered.

With QDIO, each TCP/IP instance using the device automatically registers its entire home list (including VIPAs) with OSA-Express, and OSA-Express automatically creates and initializes the OSA Address Table (OAT). There is no need for the customer to configure any home IP addresses in the OAT.

The QDIO primary and secondary routing function can provide fault tolerance without using virtual IP addresses (VIPA).

The following exampleFigure 1 illustrates how the QDIO primary and secondary routing works:

Figure 1. QDIO primary and secondary routing

                9.66.1.3
                  TCP3
               /         \
              /           \
             /             \
           TCP1           TCP2
          primary       secondary
          9.67.1.1      9.68.1.2
             \             /
              \           /
               \         /
               OSA-Express
                 (OSAX1)
                    |
                    |
                    |
              workstation W
                9.65.1.4

In this example, both TCP1 and TCP2 activate OSA-Express device OSAX1. TCP1 is defined as the primary router and TCP2 is defined as a secondary router. TCP1 registers each of its home IP addresses (just 9.67.1.1 in this example) to OSA-Express when the device is started on TCP1. Similarly, TCP2 registers each of its home IP addresses (just 9.68.1.2 in this example) to OSA-Express when the device is started on TCP2.

If device OSAX1 receives an inbound datagram destined for 9.67.1.1, then OSA-Express will route the datagram to TCP1. If the device receives a datagram destined for 9.68.1.2, then OSA-Express will route the datagram to TCP2. If the device receives a datagram destined for any other (unregistered) IP address such as 9.66.1.3, then OSA-Express will route the datagram to TCP1 because TCP1 is the primary router. If TCP1 goes down or device OSAX1 becomes inactive on TCP1, then OSA-Express will route such datagrams to TCP2. If the device subsequently becomes active again on TCP1, then OSA-Express will once again route datagrams for unregistered IP addresses to TCP1.

IP traffic from workstation W destined for 9.66.1.3 will go to TCP3 through TCP1 (because the default router is the primary router TCP1). If device OSAX1 becomes inactive on TCP1 for any reason (or TCP1 goes down), then TCP2 (secondary router) becomes the default router and the IP traffic will go to TCP3 through TCP2. If device OSAX1 becomes active again on TCP1, then TCP1 once again becomes the default router and the IP traffic will now go through TCP1.

Note: If you are using static routing, then in order for the above to work, you need to ensure that you have static routes defined in both directions between TCP3 and the workstation (9.65.1.4) through both TCP1 and TCP2.

For more information about primary and secondary routing, see https://www.ibm.com/docs/en/zos/2.3.0?topic=osa-z-systems-express-customers-guide-reference.

You can also use VIPA to provide fault tolerance. See information about Virtual IP Addressing (VIPA) in the z/OS Communications Server: IP Configuration Guide for more details on VIPA.