Why do we have a Virtual IP (VIP) in Oracle RAC 10g or 11g? 

  The goal is application availability.
  RAC 中VIP的目的是實現app應用的高可用 
  
    When a node fails, the VIP associated with it is automatically failed over to some other node.
    當rac系統中其中一個節點down掉時，被分配的vip會自動的故障轉移到健康的節點 
When this occurs, the following things happen. 
當發生failover時，會有以下動作發生

(1) VIP detects public network failure which generates a FAN（Fast Application Notification，
FAN is an event generated by the EVM） event. 
    VIP檢測網路故障產生的扇出事件   FAN事件（Fast Application Notification，FAN is an event generated by the EVM）
     
(2) the new node re-arps the world indicating a new MAC address for the IP. 
    在新的節點重新解析，表明一個新的MAC地址的IP
    
(3) connected clients subscribing to FAN immediately receive ORA-3113 error or equivalent. 
Those not subscribing to FAN will eventually time out. 
    已連線的客戶端通過訂製的FAN事件將迅速收到ORA3113錯誤，未開啟FAN事件的客戶端最終將超時
    
(4) New connection requests rapidly traverse the tnsnames.ora address list skipping over the dead nodes, 
instead of having to wait on TCP-IP timeouts 
   新的連線迅速請求通過tnsnames.ora檔案的地址列表跳過無效的節點，而不是等待TCP-IP超時
   
    Without using VIPs or FAN, clients connected to a node that died will often wait for a TCP timeout period 
(which can be up to 10 min) before getting an error. 
如果未用vip或者FAN，連線到宕機節點的客戶端在報錯之前經常會等待TCP超時

翻譯的有些牽強，歡迎拍磚