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The discovery mechanism is what makes only the address of the device required when adding a new device into Unimus.
Unimus will automatically discovered discover everything else about the device.
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The discovery before backup is necessary to make sure Unimus doesn't user use wrong commands on the device.
In certain situations (ex. when a device is changed for another device) it's possible commands to generate backups on some vendors can actually cause configuration changes for other vendors.
Due to this (and other edge-cases) Unimus does a discovery before every backup.
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