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
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).
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.
- Run Registry Editor (REGEDT32.EXE).
- From the HKEY_LOCAL_MACHINE subtree, go to the following key:
\SYSTEM\CurrentControlSet\Services\<adapter name and #>
- From the Edit menu, select Add Value.
- Add the following:
Value Name: MTU
Data Type: REG_DWORD
Data: <1500 or Ethernet segment's MTU size>
- Choose OK.
- Quit Registry Editor.
- 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. 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.