Description of Address Resolution Protocol (ARP) caching behavior in Windows Vista TCP/IP implementations

This article describes the Address Resolution Protocol (ARP) caching behavior in Windows Vista TCP/IP implementations.

ARP caching behavior has been changed in Windows Vista. The TCP/IP stack implementations in Windows Vista comply with RFC4861 (Neighbor Discovery protocol for IP version 6 [Ipv6]) for both the IPv4 and the IPv6 Neighbor Discovery process.

The ArpCacheLife and ArpCacheMinReferencedLife registry entries determine how the ARP cache is maintained in Windows XP and in Windows Server 2003. These registry entries no longer apply to Windows Vista.

In the new Windows Vista TCP/IP stack implementation, hosts create the neighbor cache entries when there is no matching entry in the neighbor cache. ARP cache entry for IPv4 is an example of a neighbor cache entry. After the entry is successfully created in the neighbor cache, the entry may change to the "Reachable" state if the entry meets certain conditions. If the entry is in the "Reachable" state, Windows Vista TCP/IP hosts do not send ARP requests to the network. Therefore, Windows Vista TCP/IP hosts use the information in the cache. If an entry is not used, and it stays in the "Reachable" state for longer than its "Reachable Time" value, the entry changes to the "Stale" state. If an entry is in the "Stale" state, the Windows Vista TCP/IP host must send an ARP request to reach that destination.

The "Reachable Time" value is calculated as follows: RFC provides the following calculated results.Therefore, the "Reachable Time" value is somewhere between 15 seconds (30 × 0.5 seconds) and 45 seconds (30 × 1.5 seconds). If an entry is not used for a time between 15 to 45 seconds, it changes to the "Stale" state. Then, the host must send an ARP Request for IPV4 to the network when any IP datagram is sent to that destination.

To see the current "Reachable Time" value, follow these steps:

1. Click Start, type cmd in the Start Search box, and then click cmd in the Programs list.
   
2. If you are prompted for an administrator password or for confirmation, type your password, or click Continue.
3. At the command prompt, type the following command:
   netsh interface ipv4 show interfaces
   Example result:

   Idx  Met   MTU   State        Name
   ---  ---  -----  -----------  -------------------
     1   50 4294967295  connected    Loopback Pseudo-Interface 1
     9   20   1500  connected    Local Area Connection
   

4. In step 2 the "Local Area Connection" Idx is 9. Therefore, you can show interface 9 by typing the following command at the command prompt:
   netsh interface ipv4 show interface 9
   Example result:

   Interface Local Area Connection Parameters
   ----------------------------------------------
   IfLuid                             : ethernet_7
   IfIndex                            : 9
   Compartment Id                     : 1
   State                              : connected
   Metric                             : 20
   Link MTU                           : 1500 bytes
   Reachable Time                     : 19000 ms Base Reachable Time : 30000 ms Retransmission Interval : 1000 ms
   				DAD Transmits : 3 Site Prefix Length : 64 Site Id : 1 Forwarding : disabled
   				Advertising : disabled Neighbor Discovery : enabled Neighbor Unreachability
   				Detecion : enabled Router Discovery : dhcp Managed Address Configuration :
   				enabled Other Stateful Configuration : enabled Weak Host Sends : disabled Weak
   				Host Receives : disabled Use Automatic Metric : enabled Ignore Default routes :
   				disabled 

5. You can change the "BaseReachable Time" value by typing the following example command at the command prompt:
   netsh interface ipv4 set interface 9 basereachable=60000
6. To see the result of step 4, type the following command at the command prompt:
   netsh interface ipv4 show interface 9
   Example result:

   Interface Local Area Connection Parameters
   ----------------------------------------------
   IfLuid                             : ethernet_7
   IfIndex                            : 9
   Compartment Id                     : 1
   State                              : connected
   Metric                             : 20
   Link MTU                           : 1500 bytes
   Reachable Time                     : 61500 ms
   Base Reachable Time                : 60000 ms Retransmission Interval : 1000 ms DAD Transmits : 3 Site Prefix
   				Length : 64 Site Id : 1 Forwarding : disabled Advertising : disabled Neighbor
   				Discovery : enabled Neighbor Unreachability Detecion : enabled Router Discovery
   				: dhcp Managed Address Configuration : enabled Other Stateful Configuration :
   				enabled Weak Host Sends : disabled Weak Host Receives : disabled Use Automatic
   				Metric : enabled Ignore Default routes : disabled 

   Note The "Base Reachable Time" value has changed to 60000 ms.
7. You can increase the neighbor cache limit by typing the following command at the command prompt:
   netsh interface ipv4 set global neighborcachelimit = 4096
   Note The default neighbor cache limit is 256.

For more information about the neighbor cache entry states, visit the following Web site:For more information about TCP/IP registry values for Windows Vista and for Windows Server 2008, visit the following Web site:The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, about the performance or reliability of these products.

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