Many techniques for redundancy can be employed in a system architecture. N redundant systems (2N, 3N, 5N, etc.) employ multiple identical sets of resources isolated into separate fault zones. N redundancy can be employed on a component level (e.g., disks and power supplies) or in complete systems (e.g., redundant file servers). This simplest form of this is 2N redundancy.

N+m redundancy involves having individual spares for a group of resources (e.g., a spare port on an Ethernet hub). The simplest form of this approach is N+1 redundancy. N+1 redundancy can also be applied to individual system components or to clusters of complete systems.

The differences in the redundancy schemes are subtle but crucial to system design. 2N redundant systems have a duplicate for every critical resource in the system. The standby resources are kept up-to-date with the activities of the active resource. When a failure occurs, the entire active domain is taken out of service, and the standby takes over. The advantages of 2N redundancy are simple fault management and fast switchover times. The disadvantages of 2N redundancy are the cost to duplicate every resource and difficulties with connectivity in systems with a large number of input/output (I/O) connections.

An example of this situation is in telephony, where a system may employ many T1/E1 connections. Duplicating the line interface boards is expensive. So is multiplexing T1s or asking the customer to lease many spare lines. For I/O intensive applications, N+1 redundancy is much more cost-effective. With N+1 redundancy, the spare cannot be fully configured because the exact nature of its task is not known until one of the active devices fails. This complicates fault management and increases switchover times.