Failover to the backup server can be set up within the System Settings of ECM for the site.
Failover Check M - How many failures accepted to occur.
Failover Check N - Number of times server status is consulted on the primary server.
Failover Check Period - Allotted time in seconds that the server status check will occur in.
Example: (Using the pic above)
If there is 1 failure out of the last 5 times the server status has been consulted, the failover will be performed.