ARP caching behavior was changed in Windows Vista, Windows Server 2008, Windows 7, Windows Server 2008 R2, Windows 8, Windows Server 2012, Windows 8.1, and Windows Server 2012 R2. The TCP/IP stack implementations in Windows comply with RFC4861 (
Neighbor Discovery protocol for IP version 6 [Ipv6]) for both the IPv4 and 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, Windows Server 2008, or a later version of a Windows-based operating system.
In the new Windows 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 TCP/IP hosts do not send ARP requests to the network. Therefore, Windows 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 TCP/IP host must send an ARP request to reach that destination.
The "Reachable Time" value is calculated as follows:
Reachable Time = BaseReachable Time × (A random value between MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR)
RFC provides the following calculated results.
| Parameter Name | Value |
|---|
| BaseReachable Time | 30,000 milliseconds (ms) |
| MIN_RANDOM_FACTOR | 0.5 second |
| MAX_RANDOM_FACTOR | 1.5 seconds |
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.
How to check the "Reachable Time" value
To see the current "Reachable Time" value, follow these steps:
- 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
- In step 1, the "Local Area Connection" Idx is 9. Therefore, you can show interface 9 by using the following command:
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
- You can change the "BaseReachable Time" value by using the following example command:
netsh interface ipv4 set interface 9 basereachable=60000
- To see the result of step 3, use the following command:
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. - You can increase the neighbor cache limit by using the following command:
netsh interface ipv4 set global neighborcachelimit = 4096
Note The default neighbor cache limit is 256.