On local area networks, ARP (address resolution protocol) is used by TCP/IP to bind a network interface's physical address to an IP address. ARP resolution, as this is called, has to be performed on a particular interface before it can communicate on the network.
To keep from having to repeatedly perform ARP resolutions, Windows 2000 stores ARP bindings in a local cache on each networked computer. The lifetime of the entries in this cache is by default a mere two minutes. If you have systems in a local network that perform a great deal of peer-to-peer communication, and their IP addresses tend to remain static for a long time (for instance, if they aren't shut down often), it makes sense to increase the lifetime of entries in the cache. Otherwise, they expire unnecessarily.
Open the Registry and navigate to
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters. Add a new DWORD entry name ArpCacheLife and set it to the total lifetime of each unreferenced cache entry in seconds. The default value is 120 seconds. The cache holds fifty entries by default, but will expand if needed.
Add another DWORD entry named ArpCacheMinReferencedLife. This governs how long an entry remains in the cache even if it has been referenced. The default value is 600 (for 600 seconds, or 10 minutes).
Another Windows 2000 ARP behavior that can be modified is the gratuitous ARP broadcast. When a Windows 2000 machine is first booted, it broadcasts an ARP packet containing that machine's TCP/IP address to make sure no other machine on the network is using the same IP address. If you are using DHCP, this is almost never needed, and can be disabled. Add or edit a DWORD entry in the same key listed above with the name ArpRetryCount and set it to 0. Be sure to reboot after making any of these changes.
By turning off gratuitous ARP broadcasts and changing the ARP cache lifetime, it's possible to significantly cut down on the amount of ARP "chatter" on a local network, and increase network performance by reducing the need for constant ARP re-resolutions.
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter. Check out his Windows 2000 blog at for his latest advice and musings on the world of Windows network administrators – please share your thoughts as well!
This was first published in June 2003