By default, WhatsUp Gold polls all devices and active monitors in your device list, unless you manually turn off polling for the system as a whole, or at the device and monitor level. The dependency feature gives you the ability to avoid turning off polling to devices, and instead makes polling dependent on the status of another device's Active Monitor(s).
Setting dependencies on one device's Active Monitors will place another device up or down depending on the type of dependency you configure.
There are two types of dependencies:
If you make devices behind a router, up dependant on the router's ping active monitor, those devices will not be polled unless the dependent router's ping attempts are successful. If the router's ping active monitor fails, the devices behind the router are placed in the unknown state. Without the dependency, the devices behind the router would fire off actions when they became unreachable due to the router's failed ping attempts. With the dependency, only actions on the router will fire.
There are several ways to "read" dependencies to ensure they are applied as you want them to be.
The map above displays several Up and Down dependencies. The green arrows indicate an Up dependency, and the red arrows indicate a Down dependency.
Using the "behind" and "in front" terminology you can follow the graphical arrow in the map above to read a dependency. For example, the server dependencies are read as, "only poll the servers if the switch is up." The servers are behind the switch, and will only be polled if the switch is also responding to polls. If the switch goes down, the server is assumed unreachable and is no longer be polled. Since the server is unreachable, the server's state then changes to Unknown.
For another example, the router dependency on the firewall is read as, "only poll the firewall if the switch is down." If a break in communication takes place between the router and the firewall, the switch changes to the Down state because it is Down dependent on the firewall. If the switch goes down, the state of the servers changes to Unknown, because they are Up dependent on the switch. Then, since the switch is down, the firewall is polled and changes to the Down state. After the firewall is considered down, the router is polled.
Down dependencies are useful in showing the break position in a chain of machines. If the chain is not broken at any point, the machines in the chain are not polled and are assumed up.