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Q140913: Communicating over TCP/IP May Fail Due to 802.3 Frame Type

Article: Q140913
Product(s): Windows for Workgroups and Windows NT Networking Issues
Version(s): 3.5,3.51,4.0
Operating System(s): 
Keyword(s): kbnetwork
Last Modified: 05-FEB-2002

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The information in this article applies to:

- Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0 
- Microsoft Windows NT Server versions 3.5, 3.51, 4.0 
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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.

Additional query words:

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Keywords          : kbnetwork 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT351search kbWinNT350search kbWinNT400search kbWinNTW350 kbWinNTW350search kbWinNTW351search kbWinNTW351 kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbWinNTS351 kbWinNTS350 kbWinNTS351search kbWinNTS350search
Version           : :3.5,3.51,4.0
Issue type        : kbprb

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