InterMapper can suppress alerts for devices that are behind or "shadowed" by another device that has failed. InterMapper cannot determine the state of these devices; they may or may not be operating, but the down equipment blocks InterMapper's "view". Put another way, 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 #2, which then connects to device #1, and then to device #3.
[InterMapper*] ----- [device#2] ------ [device#1] ------ [ device#3 ]
A Vantage Point, indicated above by the asterisk, indicates which device is the InterMapper server. If the InterMapper server does not appear on the map, place the Vantage Point on the device or network that connected downstream of the InterMapper server.
If device#2 goes down, InterMapper infers that there's no reason for reporting or alerting about the unresponsiveness of #1 and #3, because #2 is blocking their visibility by the server.
In practice, these dependencies suppress most, but not necessarily all, alerts, depending on the order in which InterMapper polls devices. Let's assume that the devices in the diagram are polled in the order 1, 2. 3. If device #2 in the diagram above actually went down while InterMapper was polling device #1, InterMapper would report device #1 as 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.
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