How do Vantage Points/Dependencies work?

Q. How do Vantage Points/Dependencies work?

A. InterMapper has the ability to suppress alerts for devices that are behind or "shadowed" by another device that has failed. These devices may or may not be operating, but InterMapper cannot determine their state because the down equipment blocks its "view". That is, the shadowed devices are "dependent" on their upstream equipment.

InterMapper uses the links between the devices on the map to determine how devices are interconnected. Thus we can see in the diagram below that InterMapper is connected to device labelled #2, which then connects to the device labelled #1, and then to #3. 

A Vantage Point indicates the icon that represents the InterMapper server on the network. In this case the asterisk at the left indicates which device is the InterMapper server. (In the event that the InterMapper server is not represented on the map, place the Vantage Point on the icon (device or network oval) that indicates the direction to the InterMapper server.

If device #2 goes down, InterMapper can infer that there's no reason for reporting or alerting about the unresponsiveness of #1 and #3, because #2 is blocking their visibility.

In practice, these dependencies will suppress most, but not necessarily all, alerts. InterMapper polls devices in an essentially random order. Let's assume that the devices in the diagram are polled in the order 1, 2. 3. If device #2 actually went down while InterMapper was polling/testing device #1, InterMapper would report that device #1 is down. It would then poll device #2, detect that it was down, and report it. Finally, it would detect that device #3 wasn't responding, but would suppress the outage because both #1 and #2 were down.