Alerts that monitor other alerts
Example: I have one router connected to the network's other router with 2 links (interfaces, tunnels, etc). If one of the links goes down the normal alert rule to email me is fine. However, if BOTH...
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You could create a service out of those two links. The service metric would be interface status. You would choose to aggregate the status data by "mean". If both links are up, they'd both return 1, so the average would be 1. If one link is down, you'd get the average of 1 and 2 (1.5). If both links are down, you'd average 2 and 2 (2). Set your threshold to >=2 and you should be good to go.
The only tedious part is setting this up for each pair of links you have.