When gratuitous ARP will occurs?

When one of the routers experiences a failure, the other router sends a Gratuitous ARP. The specific contents of the Gratuitous ARP match the packet structure described above, but the essential purpose of the packet is to let everyone on the network know that the IP 10.0.

What is gratuitous ARP?

A gratuitous ARP is a broadcast request for a router's own IP address. If a router or switch sends an ARP request for its own IP address and no ARP replies are received, the router- or switch-assigned IP address is not being used by other nodes.