If a system cannot be reached by the Vyopta data collector then it will be considered as down.
In most cases, the data collector will contact a system via the vendor-provided API, and, if a response is received the system will be considered to be "up."
However, if the attempt to connect to the API fails then the system will be flagged down.
It should be noted that the status of the system is the data collector's view - if environmental issues, such as network problems, proxy issues, firewall rule changes etc prevent collection then the Vyopta platform has no option other than marking the system as down.
Some systems are contacted via a database connection - if the credentials are incorrect (for instance a password as expired) then this may also cause a system to be marked as down since the data collector will be unable to connect to the database.