QNAP Security VioStor NVR (Version: 3.5.1) - Manual (Page 160)

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Configuration of Network Interfaces

Failover (Default settings for dual LAN NVR models)
Failover refers to the capability of switching over the network transfer port to the
redundant port automatically when the primary one fails due to hardware or connection
error to avoid network disconnection. When the primary network port resumes the
connection, the network transfer will be switched over to that port automatically.