Cisco MDS has fabric domain separation by using VSANs. Each VSAN has their own FC Domain service.
Some customers may want to segment their Windows Server traffic from Unix Traffic, or create a separate VSAN for replication.
It is a normal practice to have a VSAN extend across multiple sites with either FCIP or specialized Long Wave SFPs.
The MDS does have a feature called Inter VSAN Routing, which allows specific devices from one VSAN to communicate with a different VSAN. This typically adds a lot of complexity to the fabric, and if not documented and implemented properly, can contribute to fabric interruption. Although this feature is available, and works well, because of the complexities it can introduce I normally recommend customers re-examine the architecture to be absolutely certain this is what they really want to implement.
