Interface-layer fault-tolerance for local area networks (interface-takeover function)

The TCP/IP stack in the z/OS® Communications Server provides transparent fault-tolerance for failed (or stopped) IPv4 devices or IPv6 interfaces, when the stack is configured with redundant connectivity onto a LAN. This support is provided by the z/OS Communications Server interface-takeover function, and applies to IPv4 MPCIPA and LCS device types and to the IPv6 IPAQENET6 interface type.

At device or interface startup time, TCP/IP dynamically learns of redundant connectivity onto the LAN, and uses this information to select suitable backups in the case of a future failure of the device or interface. This support makes use of ARP flows (for IPv4 devices) or neighbor discovery flows (for IPv6 interfaces), so upon failure (or stop) of a device or interface, TCP/IP immediately notifies stations on the LAN that the original IPv4 or IPv6 address is now reachable through the backup's link-layer (MAC) address. Users targeting the original IP address will see no outage due to the failure, and will be unaware that any failure occurred.

Because this support is built upon ARP or neighbor discovery flows, no dynamic routing protocol in the IP layer is required to achieve this fault tolerance. To enable this support, you need only to configure redundancy onto the LAN:

You need redundant LAN adapters.
For IPv4, you must configure and activate multiple LINKs onto the LAN.
For IPv6, you need to configure and start multiple INTERFACEs onto the LAN.

Restriction: An IPv4 device cannot back up an IPv6 interface, and an IPv6 interface cannot back up an IPv4 device.

Rule: If static routing is used, there needs to be a static route to the LAN subnet over each interface onto the LAN. There also needs to be a default route and routes to destinations not directly attached to the LAN over each interface.

The interface-layer fault-tolerance feature can be used in conjunction with VIPA addresses, where applications can target the VIPA address, and any failure of the real LAN hardware is handled by the interface-takeover function. This differs from traditional VIPA usage, where dynamic routing protocols are required to route around real hardware failures.