Gratious ARP
Gratuitous ARP is an unsolicited message used when a host wants to tell all other nodes to update theirs ARP cache with new MAC-to-IP binding. An example of such a use-case would be when the IP address of a client is suddenly changed. Let's look at the example in Figure 1. Initially, PC1 has IP address 10.1.1.1 and has been communicating over the network for some time. Therefore Router 1 has an entry in the ARP table for PC1's IP-to-MAC binding (10.1.1.1 AAAA-AAAA-AAAA).
But what happens if suddenly, the IP address of PC1 is changed from 10.1.1.1 to 10.1.1.7? There is no way for the other nodes in the LAN including Router 1 to know that. So in the ARP cache of router 1 nothing would change until the entry 10.1.1.1 AAAA-AAAA-AAAA expires in 4 hours, but until then communication between PC1 and Router 1 would not be possible.
Gratuitous ARP has been introduced to solve this problem. When the IP address of PC1 is changed. It immediately sends out a GARP message that tells all nodes in the LAN to update their MAC-to-IP bindings with the new address 10.1.1.7 AAAA-AAAA-AAAA. Note that the message is sent unsolicitedly (without ARP Request )
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