RIP Version 2 Does Not Advertise Subnet Routes

- Microsoft Windows NT Server versions 4.0 - Routing and Remote Access Update

SYMPTOMS

Your Windows NT servers that are running the Routing and Remote Access (RRAS) Update are not advertising subnet routes for your IP networks. Instead, they are advertising one network route that summarizes the missing subnets.

CAUSE

The Windows NT routers are using a feature known as subnet hiding.

RESOLUTION

To resolve this issue, use one of the following:

• Design the network so that all subnets of a particular network ID are contiguous. For example, if you have three subnets on one network ID and one subnet from a different network ID, the three that are on the same network ID must be neighbors of each other without being separated by the fourth segment.

  -or-

• Use the same network ID throughout your connections. Then the subnet routes will always be advertised.

  -or-

• Use static routes or Open Shortest Path First (OSPF).

MORE INFORMATION

The feature described above, subnet hiding, is expected behavior with RIP version 1. RIP version 1 does not have a field for the subnet mask of a network and does not support subnet advertisements. When advertising routes to an external network that could not possibly know the subnet mask, the subnetted routes are reduced to a single network route.

RIP version 2 is capable of advertising subnet masks; however, RIP version 2 may not advertise specific subnet routes when RIP version 1 routers could mistake the routes as host routes. This would be the case if the RIP version 1 router had a different network ID than the RIP version 2 router.

This behavior of subnet hiding is always done with RIP version 1 and is usually done with RIP version 2. With some implementations of RIP version 2, subnet hiding can be enabled or disabled. Currently, the Routing and Remote Access service cannot disable subnet hiding.

STATUS

Microsoft has confirmed this to be a problem in Routing and Remote Access in Windows NT version 4.0 We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.