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Q138575: Communication Fails Through Ethernet Segment Between FDDI Rings

Article: Q138575
Product(s): Microsoft Windows NT
Version(s): winnt:3.5,4.0
Operating System(s): 
Keyword(s): kbnetwork
Last Modified: 08-AUG-2001

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The information in this article applies to:

- Microsoft Windows NT Workstation versions 3.5, 4.0 
- Microsoft Windows NT Server versions 3.5, 4.0 
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SYMPTOMS
========

The communication between two FDDI rings through an Ethernet segment may fail
when you use TCP/IP and any FDDI network interface card (NIC) in Windows NT 3.5.
For example:

  FDDI Ring-----Bridge-----Ethernet segment-----Bridge-----FDDI Ring

CAUSE
=====

Because both Windows NT 3.5 Server/Workstation are connected to the FDDI ring,
the two hosts will negotiate to use the FDDI's MTU size. However once the
communication between the two hosts begins the Ethernet segment will fail to
forward packets more than 1,500 bytes. This situation may not occur if the
bridges are replaced with routers, because router are capable of fragmenting
packets (TCP/IP only).

RESOLUTION
==========

To correct this problem, modify the registry.

WARNING: Using Registry Editor incorrectly can cause serious, system-wide
problems that may require you to reinstall Windows NT to correct them. Microsoft
cannot guarantee that any problems resulting from the use of Registry Editor can
be solved. Use this tool at your own risk.

1. Run Registry Editor (REGEDT32.EXE).

2. From the HKEY_LOCAL_MACHINE subtree, go to the following key:

  \SYSTEM\CurrentControlSet\Services\<adapter name and #>
  \Parameters\Tcpip

3. From the Edit menu, select Add Value.

4. Add the following:

  Value Name: MTU
  Data Type: REG_DWORD
  Data: <1500 or Ethernet segment's MTU size>

5. Choose OK.

6. Quit Registry Editor.

7. Shutdown and restart Windows NT.

MORE INFORMATION
================

The MTU size specifies the maximum transmission unit size of an interface. Each
interface used by TCP/IP may have a different MTU value specified. The MTU is
usually determined through negotiation with the lower driver, using that lower
driver's value. However, that value may be overridden. Ideally, the MTU should
be large enough to hold any datagram in one frame. The limiting factor is
usually the technology making the transfer. Some technologies limit the maximum
size to as little as 128; Ethernet limits transfers to 1500; and proNet-10
allows as many as 2044 octets per frame.

Datagrams larger than the MTU value are automatically divided into smaller pieces
called fragments; size is a multiple of eight octets. Fragmentation usually
occurs somewhere through which the traffic must pass whose MTU is smaller than
the encapsulated datagram. If fragmentation occurs, the fragments travel
separately to the destination computer, where they are automatically reassembled
before the datagram is processed.

Additional query words: prodnt

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Keywords          : kbnetwork 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT350search kbWinNT400search kbWinNTW350 kbWinNTW350search kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbWinNTS350 kbWinNTS350search
Version           : winnt:3.5,4.0

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