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

ID: Q140913

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

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:

Keywords : kbnetwork nttcp NTSrvWkst
Version : WinNT:3.5,3.51,4.0
Platform : winnt
Issue type : kbprb