Communication Fails Through Ethernet Segment Between FDDI Rings

• Microsoft Windows NT Workstation version 3.5
• Microsoft Windows NT Server version 3.5
• Microsoft Windows NT Server version 4.0
• Microsoft Windows NT Workstation version 4.0

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 that 1500 bits. 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.