Managing a large complex environment with ever changing operational states is challenging, to assist, NMIS as a Network Management System which is performing performance management and fault management simultaneously monitors the health and operational status of devices and creates several individual metrics as well as an over all metric for each device.  This article explains what those metrics are and what they mean.  

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Summary

Consider this in the context that a network device offers a service, the service it offers is connectivity, while a router or switch is up and all the interfaces are available, it is truly up, and when it has no CPU load it is healthy, as the interfaces get utilised and the CPU is busy, it has less capacity remaining.  The following statistics are considered part of the health of the device:

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