Internet Core Protocols: The Definitive Guide

Notes on Gratuitous ARP

As we discussed in Section 3.1.1 earlier in this chapter, one of the biggest problems that comes from devices that maintain hardware-to-IP address mappings in their local ARP caches for long periods of time is the fact that devices can and do change hardware or IP addresses. But the other devices will not be able to communicate with those systems until the ARP cache entries have expired or been replaced.

This process is particularly problematic for systems that act as servers for a network (perhaps sharing email or database services), since many clients may be trying to communicate with that system but will not be able to, due to stale ARP cache entries. Another problem that can occur quite frequently comes from dial-up clients who always negotiate a fixed IP address, regardless of the dial-up server they connect to. In that situation, the servers on the network may still have ARP cache entries for the last dial-up server that the user connected through, and will therefore be unable to communicate with the client through the new connection.

In both of these examples, stale ARP cache entries are causing fundamental communication problems, as the network devices try to send IP packets to the old hardware address associated with the destination IP address. Gratuitous ARP offers a solution to this problem by allowing devices to broadcast an ARP query for their own IP address. When this happens, other devices on the network will see the Source Hardware Address ...