Internet Core Protocols: The Definitive Guide

Cache timeout issues

Systems should flush entries from the ARP caches after they have been unused for a while. This allows well-known IP addresses to be moved to a different machine—or for a well-known machine to be given a new IP address—without communication problems coming about due to stale (and invalid) address mappings. ARP cache timeout values that are too high will cause problems whenever a host is assigned a different IP address, since the other hosts who have an older entry in their caches will still try to send data to the old (and invalid) hardware address.

Conversely, an ARP timeout that is too short will also result in problems, especially on busy networks with lots of devices. If network clients are constantly flushing their ARP caches due to short timeout values, then many broadcasts will be required. This hurts performance, since the IP software will not be able to send any data until an ARP broadcast has been sent and responded to.

RFC 826 does not give a specific timeout value for ARP cache entries, so each vendor implements this value differently. On desktop PCs, the timeout value can be quite low, with entries expiring after just 120 seconds on Windows 95 systems. Conversely, many server-class systems will set the timeout value to be ten minutes or more, while some routers set the timeout as high as four hours. These types of systems typically benefit from having high timeout values, since the client systems do not change addresses very often, and provide their ...