text: A typical application of this function is to set up a dual-site solution that uses two Storwize 
  V7000 or IBM SAN Volume Controller systems. The first site is considered the*primary site*or 
  *production site*, and the second site is considered the*backup site*or*failover site*. The failover 
  site is activated when a failure at the first site is detected. 
  
  **11.6.1 IBM Storwize family and SAN Volume Controller system layers**
  
  An IBM Storwize family system can be in one of the two layers: the*replication*layer or the 
  *storage*layer. The system layer affects how the system interacts with IBM SAN Volume 
  Controller and other external Storwize family systems. The IBM SAN Volume Controller is 
  always set to replication layer. This parameter cannot be changed. 
  
  In the storage layer, a Storwize family system has the following characteristics and 
  requirements: 
  
  (cid:2) The system can perform MM and GM replication with other storage-layer systems. 
  (cid:2) The system can provide external storage for replication-layer systems. 
  (cid:2) The system cannot use a storage-layer system as external storage. 
  
  In the replication layer, an SAN Volume Controller or a Storwize system has the following 
  characteristics and requirements: 
  
  (cid:2) The system can perform MM and GM replication with other replication-layer systems. 
  (cid:2) The system cannot provide external storage for a replication-layer system. 
  (cid:2) The system can use a storage-layer system as external storage. 