Enabling Split Horizon with Poison Reverse Causes Incorrect RIP Updates

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.

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

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:

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.

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".