SCAN virtual IP addresses (VIPs) function like node VIPs. However, unlike node VIPs, SCAN VIPs can run on any node in the cluster.
Clients (users or applications) that connect using a SCAN instead of a node VIP name or address do not have to update the list of node names or addresses in their local
tnsnames.ora file when nodes are added to or removed from the cluster, or when a database instance runs on a different node.
Configuring three SCAN VIPs in DNS does not by itself ensure failover of connections. Instead, the Oracle Client uses the returned SCAN VIPs to failover the connection request to a different SCAN listener. If the connection attempt to a SCAN VIP fails, then the client uses the next returned SCAN VIP address to connect. For this reason, Oracle recommends that you use Oracle Client 11g Release 2 or later clients for connections that use the SCAN.
If you use GNS for name resolution, then you only provide the SCAN name during installation (for example,
sales1-scan). GNS obtains DHCP address leases for three IP addresses and resolves these addresses to the SCAN. The GNS daemon listens for registrations. When a SCAN VIP starts on a node, it registers its addresses with GNS.
Service requests to the cluster domain that GNS manages are routed to the GNS VIP address, which routes these requests to the GNS daemon for the cluster. When GNS receives a request from a DNS for the SCAN, it returns the registered addresses of the SCAN listeners to the DNS. The DNS then returns the three SCAN VIP addresses to the client.
Oracle Clusterware Administration and Deployment Guide for more information about SCAN names, listeners, and client service requests