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

ID: Q240059

The information in this article applies to:

Microsoft Windows NT Server version 4.0 SP5

SYMPTOMS

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.

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/

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://www.microsoft.com/support/supportnet/overview/overview.asp

STATUS

Microsoft has confirmed this to be a problem in the Microsoft products listed at the beginning of this article. This problem was first corrected in Windows NT 4.0 Service Pack 6.

MORE INFORMATION

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."

Additional query words: 4.00

Keywords : kbbug4.00 kbfix4.00 NT4SP6Fix
Version : winnt:4.0 SP5
Platform : winnt
Issue type : kbbug