Select a multicast-capable router or Layer 3 switch as the target host to create a monitor — the SNMP service should be up and running on it. Click New Monitor, then check SNMP Custom on the Favorites tab, click Next, and confirm the host. On the next page, click Select... to open the built-in SNMP MIB Browser and type ipMRouteNextHopUpTime into the Find box to locate it in the OID tree, selecting the specific row/instance you want to monitor since this is a table column, then select it and click OK. It reports how long this multicast route's next-hop entry has been known by the router. On the monitor's Main parameters page you can set the target's SNMP port (default 161), credentials, polling interval, and other settings — see the SNMP Monitor help for details. On the State conditions and Alerting tabs, configure when the monitor should change state and trigger an alert; since this is a TimeTicks-type OID, Value bounds is the most useful condition here — trigger an alert if the value resets to a low number unexpectedly for a route you expect to be stable, since that indicates the route was lost and relearned. Click Finish to create the monitor; you can adjust any parameter later.