Configuring three IPs for SCAN listener in Oracle 11gR2

How to configure SCAN listener with DNS?

About SCAN( Single Client Access Name) Listener in Oracle 11gR2 RAC:
Single Client Access Name (SCAN) is a new Oracle Real Application Clusters (RAC) 11g Release 2 feature that provides a single name for clients to access Oracle Databases running in a cluster. The benefit is that the client’s connect information does not need to change if you add or remove nodes in the cluster. Having a single name to access the cluster allows clients to use the EZConnect client and the simple JDBC thin URL to access any database running in the cluster, independently of which server(s) in the cluster the database is active. SCAN provides load balancing and failover for client connections to the database. The SCAN works as a cluster alias for databases in the cluster.

Why three IPs to be configured for SCAN listener through DNS (Domain Name Server):

If we configure 3 IPs for SCAN listener through DNS, then in case of any failure on any SCAN IP, then fail-over will happen to other running IP. Another benefit, any client access should resolved throgh DNS also. 

Before Configuration:

$ srvctl config scan
SCAN name: prddbscan, Network: 1/101.10.1.1/255.255.255.192/en0
SCAN VIP name: scan1, IP: /prddbscan/101.10.1.4
$

Starting Configuration:
Step:1  - add three IPs in DNS , e.g.,

101.10.1.5            hrdbscan.hrapplication.com
101.10.1.6            hrdbscan.hrapplication.com
101.10.1.7            hrdbscan.hrapplication.com

Step:2 - Stop all node scan listeners

$ srvctl stop scan_listener

Step:3 - Create the below file in /etc location with adding domain name

# vi /etc/resolv.conf
search domain hrapplication.com
nameserver      101.10.9.9

Note : Name of the DNS/ AD server is "hrapplication.com" and IP is 10.10.9.9

Step: 4 - Verify with nslookup in all nodes - all node should show configured three IPs

# nslookup hrdbscan.hrapplication.com
Server:         101.10.9.9
Address:        101.10.9.9#53

Name:   hrdbscan.hrapplication.com
Address: 101.10.1.7
Name:   hrdbscan.hrapplication.com
Address: 101.10.1.6
Name:   hrdbscan.hrapplication.com
Address: 101.10.1.5

Note: If your DNS server does not return a set of 3 IPs as shown in figure 3 or does not round-robin, ask your network administrator to enable such a setup. DNS using a round-robin algorithm on its own does not ensure failover of connections. However, the Oracle Client typically handles this. It is therefore recommended that the minimum version of the client used is the Oracle Database 11g Release 2 client.

Step: 5 - modify scan
#./srvctl modify scan -n hrdbscan.hrapplication.com
#./srvctl modify scan_listener -u

-- again verify

# ./srvctl config scan 

Step: 6 - start the scan listener

#./srvctl start scan_listener
#./srvctl status scan_listener

Step: 7 - Now stop cluster services and start it again to effect

./crsctl stop crs -- one by one node

./crsctl start crs

./crsctl stat res -t
./crsctl check crs

Step: 8 - check the services

./crsctl stat res -t

HOW CONNECTION LOAD BALANCING WORKS USING SCANFor clients connecting using Oracle SQL*Net 11g Release 2, three IP addresses will be received by the client by resolving the SCAN name through DNS as discussed. The client will then go through the list it receives from the DNS and try connecting through one of the IPs received. If the client receives an error, it will try the other addresses before returning an error to the user or application. This is similar to how client connection failover works in previous releases when an address list is provided in the client connection string.

When a SCAN Listener receives a connection request, the SCAN Listener will check for the least loaded instance providing the requested service. It will then re-direct the connection request to the local listener on the node where the least loaded instance is running. Subsequently, the client will be given the address of the local listener. The local listener will finally create the connection to the database instance.

 

This document may help you. You can refer Oracle support documents for more clarification.

Case -1: Unable to start database instances after starting all cluster services:

When I tried to start database instances after starting all node cluster services, I found below error.

SQL> startup nomount;
ORA-00119: invalid specification for system parameter REMOTE_LISTENER
ORA-00132: syntax error or unresolved network name 'hrapplication.com:1521'

After some verification, I found some body disbaled below options during some RAM upgrations time.

# vi /etc/resolv.conf
#search domain hrapplication.com
#nameserver      101.10.9.9

I uncommented like below and then started my databases services and found ok.

# vi /etc/resolv.conf
search domain hrapplication.com
nameserver      101.10.9.9