Split Horizon with Poison Reverse Setting Causes Incorrect RIP Updates on Multiple Adapters

When you configure Routing Information Protocol (RIP) with multiple adapters and with all the default settings (including the Split Horizon with Poison Reverse advanced settings), the RIP updates that are sent out have the correct metrics for both of the RIP routes. However, RIP also advertises the routes that it learns on the same subnet with a metric of 16 (poison reverse). On the Routing and Remote Access Services (RRAS) computer that receives the RIP updates (with the poison reverse update), the route should not be processed because RIP already has a better route for that subnet (subnet A). It should also distinguish the fact that this update is from a computer that is not the destination router for that route to subnet A. However, RRAS processes that route and therefore discards out a good route in favor of an unreachable route (metric = 16). Any computer on its own remote subnet cannot reach this subnet (subnet A), which now has a metric of 16.

Windows NT Server or Workstation 4.0

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/downloads/details.aspx?familyid=E396D059-E402-46EF-B095-A74399E25737&displaylang=en/ (http://www.microsoft.com/Windows/ServicePacks/) -or-
  
  
• 152734  (http://support.microsoft.com/kb/152734/EN-US/ ) 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:

Windows NT Server 4.0, Terminal Server Edition

To resolve this problem, obtain the latest service pack for Windows NT Server 4.0, Terminal Server Edition. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

Microsoft has confirmed that this is a problem in Windows NT 4.0 and Windows NT Server 4.0, Terminal Server Edition.

This problem was first corrected in Windows NT Server 4.0 Service Pack 6 and Windows NT Server 4.0, Terminal Server Edition Service Pack 6.

The Split Horizon with Poison Reverse setting improves RIP convergence over a simple split horizon by advertising all network IDs, but those network IDs learned in a given direction are advertised with a metric of 16, indicating that the network is unavailable. The Poison Reverse setting has no benefit beyond the split horizon setting in a single-path internetwork. However, in a multiple-path internetwork, the Split Horizon with Poison Reverse setting greatly reduces counts to infinity and routing loops.

This behavior is described in Request for Comments (RFC) 1058, "Routing Information Protocol."