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Q195640: Split Horizon with Poison Reverse Causes Incorrect RIP Updates

Article: Q195640
Product(s): Microsoft Windows NT
Version(s): WinNT:4.0
Operating System(s): 
Keyword(s): kbWinNT400sp5fix
Last Modified: 20-MAY-2002

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The information in this article applies to:

- Microsoft Windows NT Server version 4.0 
- Microsoft Routing and Remote Access Service Update for Windows NT Server version 4.0 
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SYMPTOMS
========

When RIP is set with all the default settings, including the Advanced settings
of Split Horizon with Poison Reverse, the RIP update will occur for both bound
IP addresses but with an incorrect hop count (or metric) of 16. Because it
advertises the metrics as 16, no other routers will know that this computer is
the router between those two subnets and, thus, the two subnets will be unable
to communicate with each other.

This problem occurs on a computer that is configured to use RIP and that has a
single network adapter configured for multiple IP addresses on different
subnets.

CAUSE
=====

RRAS assigns an incorrect metric in outbound RIP requests when two IP addresses
are bound to the same network card.

RESOLUTION
==========

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or
the individual software update. For information on obtaining the latest service
pack, please go to:

- http://www.microsoft.com/Windows/ServicePacks/

-or-

- Q152734 How to Obtain the Latest Windows NT 4.0 Service Pack

For information on obtaining the individual software update, contact Microsoft
Product Support Services. 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


STATUS
======

Microsoft has confirmed this to be a problem in Windows NT version 4.0. This
problem was first corrected in Windows NT version 4.0 Service Pack 5.

MORE INFORMATION
================

Split Horizon with Poison Reverse:

Split horizon with poison reverse improves RIP convergence over simple split
horizon by advertising all network IDs, but those network IDs learned in a given
directions are advertised with a metric of 16, indicating that the network is
unavailable. Poison reverse has no benefit beyond split horizon in a single path
internetwork. However, in a multipath internetwork, split horizon with poison
reverse greatly reduces count to infinity and routing loops.

This behavior is described in RFC 1058, "Routing Information Protocol".

Additional query words: 4.00 rras multinet
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Keywords          : kbWinNT400sp5fix 
Technology        : kbWinNTsearch kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbAudDeveloper kbRRASNTSearch kbRRASNT400
Version           : WinNT:4.0
Hardware          : ALPHA x86
Issue type        : kbbug
Solution Type     : kbfix

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