Gratutitous ARP

A Gratuitous ARP is an ARP Response that was not prompted by an ARP Request. The Gratuitous ARP is sent as a broadcast, as a way for a node to announce or update its IP to MAC mapping to the entire network.

The main item to point out in the Ethernet header is the Destination MAC. Notice this frame is addressed to ffff.ffff.ffff, making it a Broadcast frame.

The Target MAC address is ffff.ffff.ffff – a reflection of the Destination MAC address. But in reality, the contents of this field are irrelevant – they are ignored in a Gratuitous ARP.


Use Cases:

This might happen if a user manually modifies their MAC address – they retain the same IP address, but now have a new MAC address. Therefore, the ARP mapping for all the nodes which are communicating with this user must be updated.

That being said, manually changing the MAC address is pretty rare. However, you do sometimes see this in redundant Cloud or Virtual environments, where a particular Virtual Machine (VM) ‘jumps’ to a new physical box – the same VM’s IP address is now being served by a different physical machine.

Our example will use two Routers sharing the IP address 10.0.0.1. The hosts in our example will be using this shared IP address as their default gateway.

When one of the routers experiences a failure, the other router sends a Gratuitous ARP.