DOCUMENT:Q244539 08-MAY-2002 [winnt] TITLE :Host Route Can Be Deleted if ICMP Fragmentation Is Needed PRODUCT :Microsoft Windows NT PROD/VER::2000,4.0 SP5,4.0 SP6 OPER/SYS: KEYWORDS:kbnetwork kbtool kbWin2000PreSP1Fix kbWinNT400PreSP7Fixkbfixlist ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Windows NT Server versions 4.0 SP5, 4.0 SP6, on platform(s): - the operating system: Microsoft Windows NT - Microsoft Windows NT Workstation versions 4.0 SP5, 4.0 SP6 - Microsoft Windows 2000 Advanced Server - Microsoft Windows 2000 Professional - Microsoft Windows 2000 Server ------------------------------------------------------------------------------- SYMPTOMS ======== When a user has defined manual host routes using the Route.exe tool, certain routes to remote hosts may be removed from the route table after 10-15 minutes after usage. CAUSE ===== This problem can occur when an ICMP Destination Unreachable Packet is sent with the DF Flag set. When this occurs, it sends along the Next Hop MTU, and this causes the Host route to be overwritten by the ICMP source route. ICMP host routes are subject to deletion if their MTU changes. Windows NT uses a Path MTU detection algorithm that sets the DoNotFragment bit on TCP packets. This behavior will cause intermediate routers with a smaller MTU than the packet's size to reject the packet and send an ICMP Destination Unreachable:Fragmentation Needed. When Windows NT receives this packet, it updates the internal routing table with the new MTU. This update would also overwrite the route's source field to ICMP. This could cause a user-entered host route to disappear when Windows NT periodically removes aged ICMP host routes. RESOLUTION ========== Windows 2000 ------------ To resolve this problem, obtain the latest service pack for Windows 2000. For additional information, please see the following article in the Microsoft Knowledge Base: Q260910 How to Obtain the Latest Windows 2000 Service Pack Windows NT 4.0 -------------- A supported fix is now available from Microsoft, but it is only intended to correct the problem described in this article and should be applied only to systems experiencing this specific problem. To resolve this problem, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web: http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS NOTE: In special cases, charges that are normally incurred for support calls may be canceled, if a Microsoft Support Professional determines that a specific update will resolve your problem. Normal support costs will apply to additional support questions and issues that do not qualify for the specific update in question. The English version of this fix should have the following file attributes or later: Date Time Size File name Platform ------------------------------------------------ 02/25/2000 07:47p 174,448 Tcpip.sys x86 02/25/2000 07:46p 310,704 Tcpip.sys Alpha WORKAROUND ========== To work around this problem, use either of the following methods: - Adjust the MTU on the router. - You can disable PMTU Discovery which will limit the MTU to 576 bytes for all non-local destination addresses. You can do this by setting this registry entry to a value of zero: Value name: EnablePMTUDiscovery Value type: REG_DWORD - Boolean Valid range: 0,1 (False, True) Default: 1 (True) Location: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters For additional information explanations of TCP registry entries, click the article number below to view the article in the Microsoft Knowledge Base: Q120642 TCP/IP and NBT Configuration Parameters for Windows 2000 or Windows NT STATUS ====== Windows 2000 ------------ Microsoft has confirmed this to be a problem in Windows 2000. This problem was first corrected in Windows 2000 Service Pack 1. Windows NT 4.0 -------------- Microsoft has confirmed this to be a problem in Windows NT 4.0. Additional query words: static persistent purge disappear table ====================================================================== Keywords : kbnetwork kbtool kbWin2000PreSP1Fix kbWinNT400PreSP7Fix kbfixlist Technology : kbWinNTsearch kbWinNTSsearch Version : :2000,4.0 SP5,4.0 SP6 Hardware : ALPHA x86 Issue type : kbbug Solution Type : kbfix ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. 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