In the hub GUI select Tunnels -> Advanced Tab: 'Tunnel is hanging Timeout' and set it to 300 seconds (5 mins).
This setting determines when the tunnel disconnect alarm is generated.
For hubs without a tunnel connection, no alarms will be sent if another hub goes down.
Another way to monitor other hubs is to use the net_connect probe to check if the NimBUS_HUB service (port 48002) is responding.