Gratuitous ARP

Gratuitous ARP

Gratuitous ARP is an ARP packet for requesting own IP address.

 Original ARP is used to resolve MAC addresses corresponding to IP addresses. What's the point of requesting your own IP address?

Gratuitous means "free of charge; out of favor; gratuitous; needless."

Is it really pointless?

The first use is to detect duplication of IP addresses due to misconfiguration, etc.

Under normal circumstances, even if you send an ARP request for your own IP address, there should be no reply from other terminals.

But what if you got a reply?

You can see that the IP address is already in use by another device.

With this mechanism, it is possible to check whether IP addresses are duplicated within the same network.

 When a Windows 2000-based computer boots up, it broadcasts a packet containing the computer's IP address within the network to prevent IP address duplication.

As a second use, VRRP also uses Gratuitous ARP.

 This is done not to check for duplicate IP addresses, but to update the ARP caches on network equipment on the same segment.

Doing so will force the update of the correspondence between IP addresses and MAC addresses.