Microsoft KB Archive/195640

= Enabling Split Horizon with Poison Reverse Causes Incorrect RIP Updates =

Article ID: 195640

Article Last Modified on 11/1/2006

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APPLIES TO


 * Microsoft Windows NT Server 4.0 Standard Edition
 * Microsoft Routing and Remote Access Service Update for Windows NT Server 4.0

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This article was previously published under Q195640



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-


 * 152734 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/directory/overview.asp



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

Keywords: kbhotfixserver kbqfe kbbug kbfix kbqfe KB195640

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