Using Default Gateways with OSPF

You must be careful when configuring a default gateway on an Open Shortest Path First (OSPF) Autonomous System boundary router. Incorrect configuration can cause loss of network connectivity to some networks.

An Autonomous System (AS) boundary router is used when external routes need to be advertised to other OSPF routers. External routes are routes learned from other routing protocols, static routes, or interfaces that are not running OSPF.

When a router is configured to be an AS boundary router, it will advertise all external routes including its default gateway. This default gateway will need to be valid for all OSPF routers in your autonomous system. An example of a bad default route is one that points to another router within the OSPF AS. This AS boundary router will end up with a default route with a next-hop gateway of itself. If this occurs, the packets destined for the default route will be dropped. If the default route is not valid for all OSPF routers, it should not be advertised. A valid default route would have the next-hop gateway address external to your OSPF AS. This route would only be configured on the router that can directly reach the external network.

One way to avoid a bad default gateway is to not use a default gateway. In many scenarios were you would be using another OSPF router as your default gateway, it is not needed. To remove the default gateway, go to Control Panel Network, click Protocols, click Properties, and remove the default gateway for each card.

Another way to avoid a bad default gateway is to create an OSPF external route filter.

1. Right-click on the OSPF icon and click Configure OSPF. If Enable Autonomous System Boundary router is selected, the External Routing tab will be displayed.
2. Click the External Routing tab, and then click the Routes button.
3. Click the Process All Routes Except Those Listed radio button. Type the address 0.0.0.0, mask 0.0.0.0, and then click Add. This will prevent this router from advertising its default gateway.