Communication Fails Through Ethernet Segment Between FDDI Rings

This article was previously published under Q138575
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The communication between two FDDI rings through an Ethernet segment mayfail when you use TCP/IP and any FDDI network interface card (NIC) inWindows NT 3.5. For example:
   FDDI Ring-----Bridge-----Ethernet segment-----Bridge-----FDDI Ring				
Because both Windows NT 3.5 Server/Workstation are connected to the FDDIring, the two hosts will negotiate to use the FDDI's MTU size. However oncethe communication between the two hosts begins the Ethernet segment willfail to forward packets more than 1,500 bytes. This situation may not occurif the bridges are replaced with routers, because router are capable offragmenting packets (TCP/IP only).
To correct this problem, modify the registry.

WARNING: Using Registry Editor incorrectly can cause serious, system-wideproblems that may require you to reinstall Windows NT to correct them.Microsoft cannot guarantee that any problems resulting from the use ofRegistry 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.
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. Sometechnologies limit the maximum size to as little as 128; Ethernet limitstransfers to 1500; and proNet-10 allows as many as 2044 octets per frame.

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

Article ID: 138575 - Last Review: 02/21/2014 00:28:08 - Revision: 2.1

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