By combining unidirectional and bidirectional topologies, you have the flexibility to create several complex topologies such as the chain and propagation topology as shown below:
In the image below there is one bidirectional replication link between Datacenter 1 and Datacenter 2 and two unidirectional replication links between Datacenter 2 and Datacenters 3 and 4. Propagation replication can be useful in a scenario when you want to setup a replication scheme between two regional offices and several other local offices. Data between the regional offices is replicated bidirectionally between Datacenter 1 and Datacenter 2. Data changes in the local offices (Datacenters 3 and 4) are pushed to the regional office using unidirectional replication:
A description of the functionality, implementation and limitations of XDCR are provided in Section 5.8.8, “Behavior and Limitations”.
To create and configure replication, see Section 5.8.5, “Configuring Replication”.