Internet Core Protocols: The Definitive Guide

Static caching

To get around some of the problems encountered with entries expiring and being over-written, many products provide tools for adding static entries to the cache that never expire. On some systems, static entries are cleared from the cache when the system is restarted, requiring they be added again, while on other systems the static entries are more or less permanent, remaining until they are manually purged or until space is needed in the cache.

There are some benefits that come from using static ARP entries. Some network administrators incorporate static entries into an overall network security strategy. By adding static ARP entries for every device on the network, an administrator can prevent someone from using somebody else’s IP address to access a sensitive host. Another benefit of static cache entries comes from the elimination of frequent lookups, since clients will always have ARP cache entries for the servers they use most frequently.

However, this benefit can also cause management headaches if systems are moved or renumbered frequently. If the network adapter is changed on a server—or if a well-known IP address is moved to another computer—then the static cache entry for that device will be invalid on all of the systems that have it. Systems will be unable to communicate until the static entries have been purged.