Probe-Based Failure Detection

The multipathing daemon performs probe-based failure detection on each interface in the IPMP group that has a test address. Probe-based failure detection involves sending and receiving ICMP probe messages that use test addresses. These messages, also called probe traffic or test traffic, go out over the interface to one or more target systems on the same local network. The daemon probes all the targets separately through all the interfaces that have been configured for probe-based failure detection. If no replies are made in response to five consecutive probes on a given interface, in.mpathd considers the interface to have failed. The probing rate depends on the failure detection time (FDT). The default value for failure detection time is 10 seconds. However, you can tune the failure detection time in the IPMP configuration file. For instructions, go to How to Configure the Behavior of the IPMP Daemon. To optimize probe-based failure detection, you must set multiple target systems to receive the probes from the multipathing daemon. By having multiple target systems, you can better determine the nature of a reported failure. For example, the absence of a response from the only defined target system can indicate a failure either in the target system or in one of the IPMP group's interfaces. By contrast, if only one system among several target systems does not respond to a probe, then the failure is likely in the target system rather than in the IPMP group itself.

Repair detection time is twice the failure detection time. The default time for failure detection is 10 seconds. Accordingly, the default time for repair detection is 20 seconds. After a failed interface has been repaired and the interface's RUNNING flag is once more detected, in.mpathd clears the interface's FAILED flag. The repaired interface is redeployed depending on the number of active interfaces that the administrator has originally set.

The in.mpathd daemon determines which target systems to probe dynamically. First the daemon searches the routing table for target systems that are on the same subnet as the test addresses that are associated with the IPMP group's interfaces. If such targets are found, then the daemon uses them as targets for probing. If no target systems are found on the same subnet, then in.mpathd sends multicast packets to probe neighbor hosts on the link. The multicast packet is sent to the all hosts multicast address, 224.0.0.1 in IPv4 and ff02::1 in IPv6, to determine which hosts to use as target systems. The first five hosts that respond to the echo packets are chosen as targets for probing. If in.mpathd cannot find routers or hosts that responded to the multicast probes, then ICMP echo packets, in.mpathd cannot detect probe-based failures. In this case, the ipmpstat -i utility will report the probe state as unknown.

You can use host routes to explicitly configure a list of target systems to be used by in.mpathd. For instructions, refer to Configuring for Probe-Based Failure Detection.