In performance load-feedback and non-load feedback you can set the target percentage for one or more data centers in a property to zero. Global Traffic Management (GTM) does not send any traffic to those data centers, except in the case where all the non-zero data centers are down: in that case, all traffic goes to whichever data centers are up, even if they have a zero target. As soon as a non-zero target data center is live again, the data centers with a zero target will no longer receive traffic. If all data centers are down, including the zero target one, and there is no backup CNAME for the property, the zero target is reinstated and traffic is directed to the data center with non-zero targets.
If a datacenter is going offline for maintenance, Akamai recommends that you check the Disabled box for that data center for the duration of the maintenance period, rather than setting its target to zero (0).
If you want to have a backup data center that will always get traffic, even when both it and the primary data centers are down, configure a failover property with that data center as its primary, and use this property as the backup CNAME for the main property.