The in.mpathd daemon considers an interface is repaired if the daemon receives responses to 10 consecutive probe packets. Also, the RUNNING flag must be set on the interface.
When an interface fails, all addresses are moved to another functional interface in the group. in.mpathd needs an address for probing so that in.mpathd can detect repairs. Consequently, you must configure a test IP address that cannot move during the failover. Moreover, you should not allow a normal application to use this test address, because the failover of network access cannot occur for these addresses. How to Configure a Multipathing Interface Group With Two Interfaces describes the steps that you perform. If in.mpathd detects a repair, failback of all network access occurs to the repaired interface.
As noted in Detecting Physical Interface Failures, automatic failback is supported for physical interfaces that are not present at system boot. See How to Recover a Physical Interface That Was Not Present at System Boot.