This article was previously published under Q128797
This article has been archived. It is offered "as is" and will no longer be updated.
When you attempt to copy large files across Digital Equipment Corporation(DEC) 250 and DEC 500 routers using the TCP/IP protocol, the file transferfails.
DEC 250 and DEC 500 are IP/DECNet Routers. For wide area networks (WAN)with DECNet as the primary protocol, the MTU size may be set to 512 bytes.These routers may not send ICMP Destination Unreachable messages withFragmentation Needed in its code field if the datagram requiresfragmentation. Instead the frame is dropped. When the Windows NTEnablePMTUDiscovery parameter is turned on (set by default), the "Don'tFragment" bit is set in the IP datagram indicating to the router that itshould not fragment this packet. At this time the router should respondwith a destination unreachable ICMP message.
To correct this problem, you must 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.
Run Registry Editor (REGEDT32.EXE).
From the HKEY_LOCAL_MACHINE subtree, go to the following key:
Value Name: EnablePMTUDiscovery Data Type: REG_DWORD Data: 0 Radix: Hex
Quit Registry Editor.
Shutdown and restart Windows NT.
The products included here are manufactured by Digital EquipmentCorporation, a vendor independent of Microsoft; we make no warranty,implied or otherwise, regarding these products' performance or reliability.