Gratuitous ARP (GARP)

I just come across new type in ARP, i.e GARP -> ARP Announcement .

This is very interesting message type,

Here i am assuming that person reading this article knowing about ARP and uses of ARP.

I am discussing about GARP called as ARP announcement.

Gratuitous ARPs (GARP) are useful for four reasons:

• They can help detect IP conflicts. When a machine receives an ARP request containing a source IP that matches its own, then it knows there is an IP conflict.
• They assist in the updating of other machines' ARP tables. Clustering solutions utilize this 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, 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.
• They inform switches of the MAC address of the machine on a given switch port, so that the switch knows that it should transmit packets sent to that MAC address on that switch port.
• Every time an IP interface or link goes up, the driver for that interface will typically send a gratuitous ARP to preload the ARP tables of all other local hosts. Thus, a gratuitous ARP will tell us that that host just has had a link up event, such as a link bounce, a machine just being rebooted or the user/sysadmin on that host just configuring the interface up. If we see multiple gratuitous ARPs from the same host frequently, it can be an indication of bad Ethernet hardware/cabling resulting in frequent link bounces. 

Example with Traffic Flow Following as,

• Two nodes in a cluster are configured to share a common IP address 192.168.1.1. Node A has a hardware address of 01:01:01:01:01:01 and node B has a hardware address of 02:02:02:02:02:02.
• Assume that node A currently has IP address 192.168.1.1 already configured on its NIC. At this point, neighboring devices know to contact 192.168.1.1 using the MAC 01:01:01:01:01:01.
• Using the heartbeat protocol, node B determines that node A has died.
• Node B configures a secondary IP on an interface with ifconfig eth0:1 192.168.1.1.
• Node B issues a gratuitous ARP with send_arp eth0 192.168.1.1 02:02:02:02:02:02 192.168.1.255. All devices receiving this ARP update their table to point to 02:02:02:02:02:02 for the IP address 192.168.1.1.

As a conclusion, GARP is mainly used for avoid IP Conflict, maintaining ARP cache entries with proper mac address applying ARP announcement, whenever interface/server/port got replied with new hardware assigning same IP Address.


Thanks.

Let me know for any clarification required.