The in.mpathd daemon sends ICMP echo probes to the targets connected to the link on all the interfaces that belong to a group to detect failures and repair. After you add an interface to a multipathing group and assign a test address, the daemon sends probes to detect failures on all the interfaces of the multipathing group."How to Configure a Multipathing Interface Group with Two Interfaces" describes the steps you perform to configure test address and groups.
Because in.mpathd determines what targets to probe dynamically, you cannot configure the targets. Routers connected to the link are chosen as targets for probing. If no routers exist on the link, arbitrary hosts on the link are chosen. A multicast packet sent to the "all hosts" multicast address (18.104.22.168 in IPv4 and ff02::1 in IPv6) determines the arbitrary hosts. The first few hosts that respond to the echo packets are chosen as targets for probing. If in.mpathd cannot find routers or hosts that responded to ICMP echo packets, in.mpathd cannot detect failures.
To ensure that each NIC in the group functions properly, in.mpathd probes all the targets separately through all the interfaces in the multipathing group. If there are no replies to five consecutive probes, in.mpathd considers the interface as having failed. The probing rate depends on the failure detection time (FDT). The default value for failure detection time is 10 seconds. The in.mpathd(1M) man page describes how to change the failure detection time. For a failure detection time of 10 seconds, the probing rate is approximately one probe every two seconds.
If five consecutive probes fail, in.mpathd considers the interface as having failed. After a failure is detected, failover of all network access takes place from the failed interface to another functional interface in the group. If you have configured a standby interface, in.mpathd chooses the standby interface for failover of IP addresses, broadcasts, and multicast memberships. If you have not configured a standby interface, in.mpathd chooses the interface with the least number of IP addresses.