Microsoft KB Archive/140913

= Communicating over TCP/IP May Fail Due to 802.3 Frame Type =

Article ID: 140913

Article Last Modified on 12/5/2003

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APPLIES TO


 * Microsoft Windows NT Workstation 3.5
 * Microsoft Windows NT Workstation 3.51
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q140913



SYMPTOMS
A Windows NT TCP/IP node is not be able to communicate to a non Microsoft based TCP/IP node.



CAUSE
The non Microsoft TCP/IP node is configured to use 802.3 SNAP encoding. Some devices can be configured to use 802.3 SNAP encoding. By default, the Microsoft TCP/IP stack transmits packets in DIX Ethernet format.



RESOLUTION
If the node device is using 802.3 SNAP encoding, modify the Windows NT registry and change the ArpUseEtherSNAP setting to 1 on the Windows NT system to match the 802.3 SNAP encoding.

Registry path:

This parameter is under the subtree HKEY_LOCAL_MACHINE under the following subkey:   \SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

Value (Parameter) Name: ArpUseEtherSNAP Data Type: REG_DWORD - Boolean Data Range: 0 or 1 (False or True) Default:   0 (False) Description: Setting this parameter to 1 will force TCP/IP to transmit Ethernet packets using 802.3 SNAP encoding. By default, the stack transmits packets in DIX Ethernet format. Windows NT always receives both formats.



MORE INFORMATION
In Windows NT 3.1, when TCP/IP is used on an Ethernet network, it will use only the Ethernet II frame format. It will recognize a SNAP formatted ARP request, but will respond with an Ethernet II formatted ARP reply in the hopes that the remote station TCP/IP stack will recognize the frame type and switch to the Ethernet II frame format.

In Windows NT 3.5, 3.51, and 4.0, there is a registry parameter (ArpUseEtherSNAP) that instructs TCP/IP to use SNAP on an Ethernet (802.3) network. With this parameter set, TCP/IP uses SNAP to encapsulate IP, ICMP, and ARP frames in 802.3 frames. If, however, a SNAP-encapsulated ARP request is responded to with an Ethernet II format ARP reply, or if a Ethernet II format ARP request is received, TCP/IP will automatically switch to using Ethernet II frames on that link.

Keywords: kbnetwork kbprb KB140913

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