High availability Cluster gratuitous failover without interrupting existing FTP and other connections

Introduction

Laymen explanation

Think about the case when you are performing a file download from an ftp server. This ftp server has failover mechanism to avoid any interruption to user and so, it claims high availablity. For this purpose, it should have deployed redundant servers which will take charge in case working server goes down. When working server fails. what do you think about FTP file download. Will it be broken? Note that FTP method creates a connection between the user machine and the working server. I wish it should not break, else I need to restart ftp download again.

Technical explanation

High availability Clustering solutions utilize gratuitous ARP when they move an IP from one NIC to another, or from one machine to another. Other machines maintain an ARP table that contains the MAC associated with an IP. When the cluster needs to move the IP to a different NIC(it happens in case of failover), be it on the same machine or a different one, it reconfigures the NICs appropriately then broadcasts a gratuitous ARP reply to inform the neighboring machines about the change in MAC for the IP. Machines receiving the ARP packet then update their ARP tables with the new MAC. So, all future packets will be delivered to new MAC. In this way, ftp connection can be saved from tear down.

Reference

http://www.taos.com/2014/07/16/understanding-gratuitous-arps/

https://support.microsoft.com/en-in/kb/244331

https://sc1.checkpoint.com/documents/R76/CP_R76_ClusterXL_AdminGuide/7292.htm