Probe-based failure detection involves the use of target systems, as described in Probe-Based Failure Detection. In identifying targets for probe-based failure detection, the in.mpathd daemon operates in two modes: router target mode and multicast target mode. In router target mode, the daemon probes targets that are defined in the routing table. If no targets are defined, then the daemon operates in multicast target mode, where multicast packets are sent out to probe neighbor hosts on the LAN.
Preferably, you should set up target systems for the in.mpathd daemon to probe. For some IPMP groups, the default router is sufficient as a target. However, for some IPMP groups, you might want to configure specific targets for probe-based failure detection. To specify the targets, set up host routes in the routing table as probe targets. Any host routes that are configured in the routing table are listed before the default router. IPMP uses the explicitly defined host routes for target selection. Thus, you should set up host routes to configure specific probe targets rather than use the default router.
To set up host routes in the routing table, you use the route command. You can use the –p option with this command to add persistent routes. For example, route -p add adds a route that will remain in the routing table even after you reboot the system. The –p option thus enables you to add persistent routes without needing any special scripts to recreate these routes with every system startup. To optimally use probe-based failure detection, make sure that you set up multiple targets to receive probes.
The procedure How to Manually Specify Target Systems for Probe-Based Failure Detection shows the exact syntax to use to add persistent routes to targets for probe-based failure detection. See Maintaining IP Connectivity and Routing While Deploying IPMP and the route(8) man page.