Distributing the default route in OSPF (Open Shortest Path First) is a useful process for directing traffic to a specific router that acts as a gateway to other networks, including the Internet.

We explain how this can be done in an OSPF configuration:

1. Define the Default Route on the Router

First, you need to make sure that the router you want to distribute the default route from has a default route configured. This is usually done by configuring a static route that points to the external gateway.

For example, on a MikroTik router or other similar device, you could configure the default route using the following command:

/ip route add dst-address=0.0.0.0/0 gateway=[gateway-IP]

2. Distribute the Default Route in OSPF

Once the default route is defined on the router, you can proceed to distribute it through OSPF. This is done by using the appropriate command to include the default route in the link state updates (LSAs) that the router sends to other OSPF routers on the network. On MikroTik routers, this is accomplished with the following command in the OSPF process:

/routing ospf instance
set [instance-name] default-route-advertise=always

This command causes the router to announce the default route to all other routers in the OSPF area. Distribution options may vary:

• always: Always announces the default route.
• if-installed: Only advertises the default route if an active default route exists in the routing table.
• never: Does not announce the default route.

3. OSPF Area Considerations

Depending on how your OSPF network is configured, you may want to control which areas the default route is distributed to. For example, it is common for the default route to be distributed in all areas from the “backbone area” (Area 0).

If you need to control distribution to specific areas, you will need to set up route filters or use specific commands that limit spread to designated areas.

4. Monitor and Verify

After configuring default route distribution, it is important to monitor the network and verify that the default route is being advertised correctly and that neighboring OSPF routers are receiving and using it.

This can be verified using diagnostic commands such as show ip route or equivalents in the router, to ensure that the default route is present in the OSPF routers' routing tables.

Distributing the default route in OSPF effectively ensures that traffic destined for external networks is routed through the intended gateway, optimizing network traffic and providing a path to external services such as the Internet.