The TCP/IP stack in z/OS® Communications Server provides transparent fault-tolerance for failed (or stopped) 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 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/interface. This support makes use of neighbor discovery flows for IPv6 interfaces, so upon failure (or stop) of an interface, TCP/IP immediately notifies stations on the LAN that the original IPv6 address is now reachable by way of the backup's link-layer (MAC) address. Users targeting the original IP address see no outage because of the failure, and they are unaware that any failure occurred.

Because this support is built upon neighbor discovery flows, no dynamic routing protocol in the IP layer is required to achieve this fault tolerance. To enable this support, you must configure redundancy onto the LAN by defining and activating multiple INTERFACEs onto the LAN. Note that an IPv4 device cannot back up an IPv6 interface, and an IPv6 interface cannot back up an IPv4 device.

The interface-layer fault-tolerance 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 true hardware failures.