Metro/Global Copy relationships are established, suspended, and terminated for each pair separately. You should first establish the remote B-C pair, specifying the CASCADE (YES) option and the MODE (NOCOPY) options. It is recommended that CGROUP(YES) not be specified when establishing this path, since consistency groups are not applied to remote relationships. Then establish the local A-B pair, specifying the OPTION (SYNC) and MODE (COPY) options. Other options depend on your specific environment.
In order to ensure a tier four data loss solution, periodic synchronization of the B-C pair is required to complete all updates to the remote secondary. This is accomplished by freezing updates at volume A and determining that volume C is in a consistent state with that point in time, either by waiting for all updates to be copied from volume B, or by converting the PPRC-XD pair to a synchronous relationship so that all pending updates are processed sequentially. (Synchronizing the PPRC-XD pair can have a significant impact on performance and should be scheduled with care.) When all updates to volume C have completed, make a FlashCopy® or tape backup, resume I/O operations at the local pair and restore the remote pair to its asynchronous state. Scenarios using PPRC extended distance and synchronous mode for backup purposes explains this procedure in greater detail.