Internet Core Protocols: The Definitive Guide

Gratuitous ARP

Simply put, Gratuitous ARP is where a device broadcasts its own hardware and IP address pair, solely for the purpose of causing other devices to update their ARP caches. Remember that systems will not add new devices that they see in a broadcast (as this may cause the cache to flush more important entries). However, they will gladly update any entries that are already in the cache.

When a Gratuitous ARP request is broadcast, the sender puts its hardware and IP address information into the appropriate sender fields, and also places its IP address in the destination IP address field. It does not put its hardware address in the destination hardware field, however. Other devices on the network see this broadcast, and if they have the sender’s information in their cache already, they either restart the cache entry’s countdown timers or modify the entry to use the new hardware address (in case the network adapter on the sending host has been changed).

This process can be particularly useful with servers that communicate with many different clients frequently; the frequent use of Gratuitous ARP messages causes those clients to update their cache entries frequently, thereby keeping the clients from having to constantly reissue queries whenever they want to connect to this server. Furthermore, Gratuitous ARP messages can also be useful when a system frequently dials into a modem pool that is served by multiple communications servers, and the user has pre-selected an IP address ...