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After Oracle RAC 11.2 comes SCAN (single client access name) for RAC database.
SCAN IPs must be 3, with Round-Robin alghoritm.
Then SCAN Listeners must be 3.
Mahir M. Quluzade
GregG wrote:The number of SCAN IP does not depend on qtd of node, because the SCAN IP is a "cluster resource" which can be in any place of cluster.
Oracle recommends min 3 SCAN listeners , but what if I've got 2 node RAC ? :)
But if you have a cluster with only 2 nodes, in this particular case 2 SCAN IP is enough ... I see no benefits to use 3 SCAN IP in a cluster with 2 nodes.
Use 3 SCAN IP is recommended not mandatory.
IMHO, The setup of 3 SCAN IP have benefit when you have 3 or more nodes on your clusterware ... I see no benefit duplicate the same "cluster resource" (SCAN IP/Listener) on the same node.
About SCAN :
1. The single client access name (SCAN) is the address used by all clients connecting to the cluster.
2. The SCAN name is a domain name registered to three IP addresses, either in the domain name service (DNS) or the Grid Naming Service (GNS).
3. The SCAN name eliminates the need to change clients when nodes are added to or removed from the cluster.
4. Clients using SCAN names can also access the cluster using EZCONNECT.
5. The Single Client Access Name (SCAN) is a domain name that resolves to all the addresses allocated for the SCAN name.
6. Three IP addresses should be provided (in DNS) to use for SCAN name mapping as this ensures high availability.
7. During Oracle Grid Infrastructure installation, listeners are created for each of the SCAN addresses, and Oracle Grid Infrastructure controls which server responds to a
SCAN address request.
8. The SCAN addresses need to be on the same subnet as the VIP addresses for nodes in the cluster.
Note 1 : SCAN IPs must NOT be in the /etc/hosts file, the SCAN name must be resolved by DNS.
Note 2 : When you are installing Grid Infrastructure it will ask for the following things :
*1. Cluster Name :*
*2. Scan Name :*
*3. Scan Port :*
3 SCAN addresses regardless of number of cluster nodes.
http://lmgtfy.com/?q=oracle+scan+pdf and read it for yourself.
Really, I installing RAC every time with 3 SCAN IP. I didn't try use 1 or 2 scan IP address. I will try :)
Three Static IP addresses configured on the domain name server (DNS) before installation so that the three IP addresses are associated with the name provided as the SCAN, and all three addresses are returned in random order by the DNS to the requestor
Mahir M. Quluzade