Network Security Blog

1)Both source and destination IP in the packet are the IP of the host issuing the gratuitous ARP
2)The destination MAC address is the broadcast MAC address (ff:ff:ff:ff:ff:ff)
This means the packet will be flooded to all ports on a switch
    No reply is expected

Gratuitous ARP is used for some reasons:

     1)Update ARP tables after a MAC address for an IP changes (failover, new NIC, etc.)

     2)Update MAC address tables on L2 devices (switches) that a MAC address is now on a different port

     3)Send gratuitous ARP when interface goes up to notify other hosts about new MAC/IP bindings in advance so that they don't have to use ARP requests to find out

PC-----10.10.10.100(PA)

GARP request:-<-----BROADCAST

    No reply is expected

If reply comes then it means  that you have an IP address conflict in your network

GARP reply<----UNICAST

source and destination IP are both set to the IP of the machine, which is issuing the packet and the target MAC is the sender MAC.