XDCR advanced topologies
By combining unidirectional and bidirectional topologies, you have the flexibility to create several complex topologies such as the chain and propagation topology.
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: