Gratuitous ARP enables a host to send an ARP Request packet using its own IP address as the destination address. Gratuitous ARP provides the following functions:
Checks duplicate IP addresses: Normally, a host does not receive an ARP Reply packet after sending an ARP Request packet with the destination address being its own IP address. If the host receives an ARP Reply packet, another host has the same IP address.
Advertises a new MAC address. If the MAC address of a host changes because its network adapter is replaced, the host sends a gratuitous ARP packet to notify all hosts of the change before the ARP entry is aged out.
Notifies an active/standby switchover in a VRRP backup group: After an active/standby switchover, the master switchsends a gratuitous ARP packet in the VRRP backup group to notify the switchover.