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Address Resolution Protocol (ARP) Cache Functions

This article was previously published under Q99150
Retired KB Content Disclaimer
This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.
This article discusses the ARP cache, how it manages its resources andits problem with duplicate IP addresses.


The address resolution protocol (ARP) cache is a table in computermemory that maps a limited number of IP addresses to their physicaladapter addresses. A computer's ARP cache contains its own entry,entries for machines that have made ARP broadcasts to it, and entriesfor machines to which it has made broadcasts.


ARP cache size is controlled by the "arptblsize" parameter in the[tcp_xif] section of the PROTOCOL.INI file. The default is(tcpconnections*2)+6. The range is from 6 to 512.


Each cache entry has a time stamp that the ARP uses to control itsresources. To make more room for active entries, the ARP walks throughthe cache every five minutes and deletes any entry that is more than15 minutes old and is not for a local machine or default gateway. Ifthe cache becomes full, the ARP clears outdated or remote entriesusing the same criteria. The time stamp and the maximum time allowedin the cache depend on the implementation and are not to be confusedwith an IP packet's "time to live" parameter. The cache is flushedwhen the machine reboots.


Duplicate IP addresses can cause ARP problems. Here is an example thatshows how:

Machine A's ARP cache has Ethernet address E1 entered for machine B.Machine C starts up, and it has the same IP address of machine B butan Ethernet address of E2. Machine A's ARP cache is updated to reflectthe new Ethernet address for that IP address, which can cause seriousproblems (including hanging the machine) if A is already in a sessionwith B.

To avoid this problem, you have to plan the network so that eachmachine has a unique IP address.

Microsoft Product Support Services provides a utility for looking atARP cache contents.
LAN Manager Resource Kit, pp. 7-19 to 7-21.
2.20 arp 2.2

Article ID: 99150 - Last Review: 02/19/2002 16:31:00 - Revision: 1.0

  • KB99150