Failover is the process in which a target stands in for a failed source. As a result, user and application requests that are directed to the failed source are routed to the target. When a monitored source misses a user-defined number of requests it assumes that the machine has failed. The system then prompts the network administrator to initiate failover, or, if configured, it occurs automatically. The failover target assumes the network identity of the failed source. When the target assumes the identity of the source, user and application requests destined for the source machine or its IP address(es) are routed to the target. When partnered with the replication capabilities, failover routes user and application requests with minimal disruption and little or no data loss. In some cases, failover may be used without data replication to ensure high availability on a machine that only provides processing services, such as a web server.
Failover can be configured to stand in for one or more IP addresses associated with different NICs on the source. Each IP address can be added to a specific target NIC making NIC configuration very flexible. For example, a single NIC on the source may have one or more IP addresses assigned to it. If that source or the NIC fails, all traffic from the source is directed to the target. If there are multiple NICs on the source, the target can assume the traffic from all of the addresses. Additional NICs on the target increase flexibility and control. Secondary target NICs can assume the traffic from a failed source NIC while normal target traffic can continue to use the primary target NIC.

NOTE: Failover is not available in Replication Editions, including the Small Business Server Edition. You will see SBS servers in the Failover Control Center, but you will not be able to configure them for failover. Use Full-Server Failover for failover of SBS servers. Contact us for more details.