What is Virtual IP and how it works in Oracle RAC

In real application Cluster environment set up following are the IPs required per Node.

1. Private IP: This IP is used for Node interconnection. Systems can't be access using this IP from outer world.

2. Public IP: This IP is to used for accessing system for day to day tasks monitoring etc.

3. Virtual IP (VIP): This IP is required for fail over in case of Node is down. This will move to surviving node.

In Database Management Software Oracle 11g R2, One more concept is introduced Single Client Access Name (SCAN). Whole Real Application Cluster has given a name called SCAN name. This
is basically a name corresponds to minimum one or recommended three IP's. These IP's are called SCAN VIP's.

Let's see how VIP works 

Suppose, we have a two node Real Application Cluster set up with following IP's

NODE              Static IP address                     Virtual IP address
=======================================================================
racnode1            192.168.1.100                         192.168.1.200
                               (racnode1)                           (racnode1_vip1)

racnode2           192.168.1.101                          192.168.1.201
                              (racnode2)                            (racnode2_vip2)

In Database Management Software Oracle 10g:

Let's first see how this works in Oracle 10g. Suppose, Listener.ora of both Database is using Static IP for it's configuration like

LISTENER=
  (DESCRIPTION=
    (ADDRESS_LIST=
      (ADDRESS=(PROTOCOL=tcp)(HOST=racnode1)(PORT=1521))
      (ADDRESS=(PROTOCOL=ipc)(KEY=extproc))))
SID_LIST_LISTENER=
  (SID_LIST=
    (SID_DESC=
      (GLOBAL_DBNAME=sales.us.example.com)
      (ORACLE_HOME=/oracle10g)
      (SID_NAME=Service1))
    (SID_DESC=
      (SID_NAME=plsextproc)
      (ORACLE_HOME=/oracle10g)
      (PROGRAM=extproc)))

Hence, Tnsnames.ora for Client system will be like

Service1 =
(DESCRIPTION =
  (ADDRESS=(PROTOCOL=TCP)(HOST=racnode1)(PORT=1521))
  (ADDRESS=(PROTOCOL=TCP)(HOST=racnode2)(PORT=1521))
    (CONNECT_DATA =
      (SERVICE_NAME = Service1)
     )
  )

Now, A new connection to database will first go to racnode1, if this node is alive and working fine connection will be establish and user can continue work.

What if, racnode1 is not available Even in this case, client tries to establish a connection with the racnode1 Because, it is first in its address list.But since the node(racnode1) is not available, client tries to establish it’s connection with the next available address in the list (i.e racnode2). So, there is a delay to move from one node to other. This is called Connect-Time Failover.

But the Problem is that the TIME (TCP TIMEOUT) it takes to failover, which will be ranging between a few seconds to a few minutes. For a very high critical systems/environments this is not acceptable.

To resolve this problem Oracle introduce Virtual IP (VIP).

Let's see how it works with VIP

Now, Listener.ora of both Database is using VIP for it's configuration like

LISTENER=
  (DESCRIPTION=
    (ADDRESS_LIST=
      (ADDRESS=(PROTOCOL=tcp)(HOST=racnode1_vip1)(PORT=1521))
      (ADDRESS=(PROTOCOL=ipc)(KEY=extproc))))
SID_LIST_LISTENER=
  (SID_LIST=
    (SID_DESC=
      (GLOBAL_DBNAME=sales.us.example.com)
      (ORACLE_HOME=/oracle10g)
      (SID_NAME=Service1))
    (SID_DESC=
      (SID_NAME=plsextproc)
      (ORACLE_HOME=/oracle10g)
      (PROGRAM=extproc)))

Hence, Tnsnames.ora for Client system will be like

Service1 =
(DESCRIPTION =
  (ADDRESS=(PROTOCOL=TCP)(HOST=racnode1_vip1)(PORT=1521))
  (ADDRESS=(PROTOCOL=TCP)(HOST=racnode1_vip2)(PORT=1521))
    (CONNECT_DATA =
      (SERVICE_NAME = Service1)
     )
  )

Now, A new connection to database will first go to racnode1_vip1, if this node is alive and working fine connection will be establish and user can continue work.

What if, racnode1_vip1 is not available Even in this case, client tries to establish a connection with the racnode1_vip1 Because, it is first in its address list. But since the node(racnode1_vip1) is not available, CRS will come in to picture and move the failed node’s VIP to one of the surviving nodes of the cluster.

 Any connection attempts to the failed node by using VIP will be handled by the failed node’s VIP that is currently residing on one of the surviving node.

This (failed node’s VIP) will respond immediately to client by sending an error indicating that there is no listener. Upon receiving the information of no listener,client immediately retry connection using the next IP in the address list. Thus reduces the time to failover.
In Database Management Softwere Oracle 11g2:

When we talk about Oracle 11g, Since, we have SCAN VIP's in Oracle 11g, Following question comes into mind 
 
Do we still need VIP in Oracle 11g ?

 Yes, We still need VIP. VIP still play the same role as it is discussed in case of Database Management Softwere Oracle 10g.

 What is the Difference between SCAN VIP and VIP ?

The IP address corresponding to SCAN NAME are called as SCAN VIP. Which runs on DB nodes as SCAN LISTENERS.

 Let's see how VIP's works in 11g R2.  In Oracle 11g R2, tnsnames.ora will have only one entry that is scan name of the Cluster like.

Service1 =
(DESCRIPTION =
  (ADDRESS=(PROTOCOL=TCP)(HOST=scan_racnode1_vip1)(PORT=1521))
    (CONNECT_DATA =
      (SERVICE_NAME = Service1)
     )
  )

This scan name is resolved by any of the SCAN VIP and every SCAN VIP has a Listener associated with it running on node know as SCAN LISTENER. In below example, There are two SCAN LISTENER's running on odain1 and odain2.

[grid@bin]$ srvctl status scan_listener
SCAN Listener LISTENER_SCAN1 is enabled
SCAN listener LISTENER_SCAN1 is running on node odain1
SCAN Listener LISTENER_SCAN2 is enabled
SCAN listener LISTENER_SCAN2 is running on node odain2

All databases are registered with each SCAN LISTENER in the Cluster and PMON updates it's load to each SCAN LISTENER. Each request go through using SCAN_NAME, resolves to SCAN VIP i.e. SCAN LISTENER. Now, SCAN LISTENER redirects it to VIP by deciding using Load Balance.

SCAN _NAME ===============> SCAN VIP ==============> VIP