Internet Core Protocols: The Definitive Guide

Cache size issues

Most systems have a very limited ARP cache, with only enough room for a few entries. These entries will be overwritten as needed—a problem for busy networks.

For example, if a client system frequently accesses a variety of different servers (one for mail, one for web, one for routing, one for database, etc.), and its ARP cache was smaller than the number of systems it was connecting to, then it would have to overwrite entries in the cache on a continual basis. It would issue an ARP broadcast, cache the result, and then overwrite that entry with the results from another ARP broadcast for a different system a few moments later. If this problem were multiplied by hundreds of devices, the network could theoretically become saturated with ARP broadcasts.

Likewise, if many clients are accessing a single server, and if the ARP cache on that server is too small to hold entries for all of the clients, then the ARP cache on the server may get flushed continually. This would require the server to issue ARP requests whenever it needed to send data back to the clients. This in turn would flush even more entries, forcing the cycle to be repeated endlessly.

Note that many large, multiuser systems and network routers have very large ARP caches to prevent these types of problems from occurring in the first place. For example, high-end Cisco routers have an ARP cache that is large enough to hold several hundred entries, since the router is likely to exchange data with each PC on the ...