How does it work?
In this solution, each NVR has a reserved number of channel capacity in order to act as a redundancy server whenever necessary. For example, there is a 256-channel INR-series NVR with 100 primary channels. That leaves an additional 156-channel capacity to act as a redundancy server for another NVR.
If the other NVR with 100 channels becomes unreachable, the redundancy NVR will now manage both its original cameras and the unreachable NVR's cameras. When the unreachable NVR recovers itself, the redundancy NVR will sync and hand over the cameras back to the original service.
This method also provides flexibility - you can decide which camera channels you want to serve under redundancy mode. You can skip less important channels, for example. However, if the full redundancy must be achieved in the network, then on average each NVR should have twice as many video channel licenses activated than there are cameras under that NVR.