Gratuitous ARP

Gratuitous ARP

Function:

1,ARP refreshes other device's ARP caches after reboot.

2,ARP will look for duplicate IP addresses.

 

Gratuitous ARP is an ARP packet sent by a node in order to spontaneously cause other nodes to update an entry in their ARP cache. A gratuitous ARP may use either an ARP request or an ARP reply packet. In either case, the ARP sender Protocol address and ARP target protocol address are both set to the IP address of the cache entry to be updated, and the ARP sender hardware address is set to the Link-Layer address to which this cache entry should be updated.When useing an ARP reply packet, the target hardware address is also set the Link-Layer address to which this cache entry should be updated(this field is not used in an ARP request packet).

Most hosts on a network will send out a Gratuitous ARP when they are initialising their IP stack. This gratuitous ARP is an ARP request for their own IP address and is used to check for duplicate IP address.If there is a duplicate address then the stack does not complete initialisation.