Flex Clusters
Oracle 12c support two types of cluster configuration at the time of Clusterware installation: Traditional Standard Cluster and Flex cluster. In a traditional standard cluster, all nodes in a cluster are tightly integrated to each other and interact through a private network and can access the storage directly. On the other hand, the Flex Cluster introduced two types of nodes arranged in Hub and Leaf nodes architecture. The nodes arranged in Hub nodes category are similar to the traditional standard cluster, i.e. they are interconnected to each other through a private network and have the directly storage read/write access. The Leaf nodes are different from the Hub nodes. They don’t need to have direct access to the underlying storage; rather they access the storage/data through Hub nodes.
You can configure Hub nodes up to 64, and Leaf nodes can be many. In an Oracle Flex Cluster, you can have Hub nodes without having Leaf nodes configured, but no Leaf nodes exist without Hub nodes. You can configure multiple Leaf nodes to a single Hub node.  In Oracle Flex Cluster, only Hub nodes will have direct access to the OCR/Voting disks.  When you plan large scale Cluster environments, this would be a great feature to use. This sort of setup greatly reduces interconnect traffic, provides room to scale up the cluster to the traditional standard cluster.
There are two ways to deploy the Flex Cluster:
  1. While configuring a brand new cluster
  2. Upgrade a standard cluster mode to Flex Cluster
If you are configuring a brand new cluster, you need to choose the type of cluster configuration during step 3, select Configure a Flex Cluster option and you will have to categorize the Hub and Leaf nodes on Step 6. Against each node, select the Role, Hub or Leaf, and optionally Virtual Hostname too.

The following steps are required to convert a standard cluster mode to Flex Cluster mode:
1. Get the current status of the cluster using the following command:
$ ./crsctl get cluster mode status
2. Run the following command as the root user:
$ ./crsctl set cluster mode flex
$ ./crsctl stop crs
$ ./crsctl start crs
3. Change the node role as per your design

$ ./crsctl get node role config
$ ./crsctl set node role hub|leaf

$ ./crsctl stop crs-wait

$ ./crsctl start crs
Note the following:
  1. You can’t revert back from Flex to Standard cluster mode
  2. Cluster node mode change requires cluster stack stop/start
  3. Ensure GNS is  configured with a fixed VIP
OCR backup in ASM disk group
With 12c, OCR can be now be backed-up in ASM disk group. This simplifies the access to the OCR backup files across all nodes. In case of OCR restore, you don’t need to worry about which node the OCR latest backup is on. One can simply identify the latest backup stored in the ASM from any node and can perform the restore easily.
The following example demonstrates how to set the ASM disk group as OCR backup location:
$ ./ocrconfig -backuploc +DG_OCR
IPv6 support
With Oracle 12c, Oracle now supports IPv4 and IPv6 network protocol configuration on the same network. You can now configure public network (Public/VIP) either on IPv4, IPv6 or combination protocol configuration. However, ensure you use the same set of IP protocol configuration across all nodes in a cluster.