Gratuitous ARP – How Gratuitous ARP Works

When in doubt, go to the source, RFC 826, An Ethernet Address Resolution Protocol

1. There are two type of ARP messages: REQUEST and REPLY.

2. The RFC specifies the ethernet header. An ARP request involves determining the ethernet address of the destination, so it uses the broadcast address since it doesn't know the unicast address. An ARP reply will know the ethernet address, so it will be a unicast.

   Ethernet transmission layer (not necessarily accessible to the user):  
       48.bit: Ethernet address of destination  
       48.bit: Ethernet address of sender  
       16.bit: Protocol type = ether_type$ADDRESS_RESOLUTION
   

   It then causes this packet to be broadcast to all stations on the Ethernet cable originally determined by the routing mechanism.

3. The way the ARP cache works is OS-dependent, and there is nothing requiring a host to maintain an ARP cache.

4. The use of gratuitous ARP is not officially documented, but some hosts and OSes use it to do things like resolve address conflicts; Wireshark has a pretty good explanation (with examples): Gratuitous ARP

Gratuitous ARP could mean both gratuitous ARP request or gratuitous ARP reply. Gratuitous in this case means a request/reply that is not normally needed according to the ARP specification (RFC 826) but could be used in some cases. A gratuitous ARP request is an AddressResolutionProtocol request packet where the source and destination IP are both set to the IP of the machine issuing the packet and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff. Ordinarily, no reply packet will occur. A gratuitous ARP reply is a reply to which no request has been made.

It can just put the packet on the interface through which it can reach the other router.

There are problems with your premise. Remember that frames on a link are delivered via a layer-2 protocol, and that protocol may have an address that must be resolved in order to encapsulate the packets into frames. If the frames use MAC addresses, then you need ARP to resolve the next-hop layer-3 address into a layer-2 address.

Also, if the link is a multiaccess link, how does the router direct the packets to the one-of-many hosts on the multiaccess link? The packets only have the source and destination layer-3 addresses, not any of the intermediate hop addresses. Yes, the router knows that the packets must go out specific interfaces, but it must have some way to direct the frames to the specific device (router) on a multiaccess interface.

Yes, there are layer-2 protocols that may use other, or no, addressing on the frames. If there is no addressing, e.g. PPP, then the router knows how to create the frame, and there is only one other device on the link, but there are many other layer-2 protocols (frame relay, ATM, etc.) that use other addressing, and a router will use a method other than ARP to resolve the layer-2 address of the next hop.

When ARP is used by a host in a LAN, then it is broadcast because the host does not know where is the destination host, but a router knows which interface it must use to get to the next router.

A router also knows which interface a host is connected on. Remember that at layer-2, another router is just a host on a network. The router knows nothing more about another router than it does a host.

Also, only an ARP request is broadcast. ARP also specifies that a host create an ARP table, and ARP may simply find the layer-2 address in the ARP table without using broadcast.