The network interface cards used in all the diagrams in this section have been OSA-Express cards. There is so much capability built into an OSA-Express card that it warrants a significant portion of hardware connectivity on the mainframe. It is appropriate though, to mention here some of the cooperation that occurs between TCP/IP and OSA-Express, and how this can effect availability within a sysplex environment.

The first point to note is that an OSA-Express card (when configured appropriately) automatically receives a copy of all IP addresses that are active within a TCP/IP stack. When a DVIPA is added to or removed from a TCP/IP stack, the OSA-Express card is immediately informed of the change.

The second point of note is that for simplicity's sake, the figures in this section have all shown a single OSA-Express card attached to each individual LPAR. Each OSA-Express card, however, has two physical connectors (adapters) available. And, an OSA-Express card can be shared with more than one LPAR (within a CPC, of course). As a result, most implementations of an OSA-Express card involve sharing the OSA-Express card between two LPAR's.

In Figure 1, the OSA adapters have been pulled away slightly from the LPARs in order to show the cross-adapter redundancy of sharing OSA adapters. The presumption here is that all adapters are connected to the same LAN segment and the same subnetwork.

If we imagine that the OSA-E2 card were to fail, we can readily see that both LPARs would maintain complete connectivity to the LAN using OSA-E3. What is not so apparent is the gratuitous ARP processing. Gratuitous ARP processing is part of the availability features of the OSA-Express card.

The end result is that even though IP addresses 201.2.11.1 (LPAR 1) and 201.2.11.4 (LPAR 2) appear to be lost when OSA-E2 is lost, they are immediately revived in the OSA-E3 card by a gratuitous ARP updating the MAC addresses of the lost IP addresses.

In other words, network connectivity is re-established by making changes at layer 2 (link layer) instead of layer 3 (IP layer). The routers in the network would effectively not be involved and routing tables remain unchanged.

Figure 1. Shared OSA adapters in a sysplex