The only difference is that , we specify the scan name instead of all VIPs in the tnsnames.ora file.
So what is the benifit with it?
Suppose if you are adding any node to the cluster, you need to specify corresponding VIP entry for that node in the tnsnames.ora prior to 11gR2. But with SCAN name, we dont need to do any modifications to the tnsnames.ora file. SCAN name will resolve and load balance the client connections to all the nodes in the RAC.
SCAN configuration in the cluster :
The SCAN is configured during the installation of the grid infrastructure that is distributed with Oracle Database 11g Release2. So in order to configure SCAN, you need to create a single name that resolves to 3 IP addresses using a round robin algorithm. The IP addresses must be on the same subnet as your public network in the cluster.
rac-scan.example.com IN A 220.127.116.11
IN A 18.104.22.168
IN A 22.214.171.124
Hope it helps