When an underlying interface fails and probe-based failure detection is used, the in.mpathd daemon continues to use the interface's test address to continue probing target systems. During an interface repair, the restoration proceeds depending on the original configuration of the failed interface:
Failed interface was originally an active interface – the repaired interface reverts to its original active status. The standby interface that functioned as a replacement during the failure is switched back to standby status if enough interfaces are active for the group as defined by the system administrator.
An exception to this step are cases when the repaired active interface is also configured with the FAILBACK=no mode. For more information, see The FAILBACK=no Mode
Failed interface was originally a standby interface – the repaired interface reverts to its original standby status, provided that the IPMP group reflects the original number of active interfaces. Otherwise, the standby interface is switched to become an active interface.
To see a graphical presentation of how IPMP behaves during interface failure and repair, see How IPMP Works.
By default, active interfaces that have failed and then repaired automatically return to become active interfaces in the group. This behavior is controlled by the setting of the FAILBACK parameter in the daemon's configuration file. However, even the insignificant disruption that occurs as data addresses are remapped to repaired interfaces might not be acceptable to some administrators. The administrators might prefer to allow an activated standby interface to continue as an active interface. IPMP allows administrators to override the default behavior to prevent an interface to automatically become active upon repair. These interfaces must be configured in the FAILBACK=no mode. For related procedures, see How to Configure the Behavior of the IPMP Daemon.
When an active interface in FAILBACK=no mode fails and is subsequently repaired, the IPMP daemon restores the IPMP configuration as follows:
The daemon retains the interface's INACTIVE status, provided that the IPMP group reflects the original configuration of active interfaces.
If the IPMP configuration at the moment of repair does not reflect the group's original configuration of active interfaces, then the repaired interface is redeployed as an active interface, notwithstanding the FAILBACK=no status.
The FAILBACK=NO mode is set for the whole IPMP group. It is not a per-interface tunable parameter.