OSPF Default Route

In most enterprise networks, only one router is connected to the Internet. This router knows how to reach all external destinations.

Instead of configuring a static default route (0.0.0.0/0) on every internal router, we can define it once on the edge router (R1) and then inject it into the OSPF domain.

Figure 1 - OSPF used to demonstrate OSPF Default Route

This approach ensures that internal routers like R2 and R3 can dynamically learn the default route via OSPF, and forward any unknown traffic toward R1, which acts as the gateway to the Internet.

First, we need to configure the default route using ip route 0.0.0.0 pointing to the IP of the ISP router.
In our scenario, the next-hop IP address is 203.0.115.2.

We assume OSPF neighbor relationships are already established and that networks are being advertised in Area 0.

Figure 2 - Configuration of default static route on R1

We now configure the static default route on R1:

R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.115.2

This instructs R1 to send all unknown destination traffic to the ISP router at 203.0.115.2.

To verify that the route has been installed correctly, you can inspect the routing table:

R1# show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is 203.0.115.2 to network 0.0.0.0

     192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.1.0/24 is directly connected, GigabitEthernet0/1
L       192.168.1.1/32 is directly connected, GigabitEthernet0/1
     192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.2.0/24 is directly connected, GigabitEthernet0/0
L       192.168.2.1/32 is directly connected, GigabitEthernet0/0
     203.0.115.0/24 is variably subnetted, 2 subnets, 2 masks
C       203.0.115.0/24 is directly connected, GigabitEthernet0/2
L       203.0.115.1/32 is directly connected, GigabitEthernet0/2
S*      0.0.0.0/0 [1/0] via 203.0.115.2

In this output:

• The letter S indicates a static route

• The asterisk * indicates that this route is the candidate default route

• The line Gateway of last resort... confirms that R1 is now set to forward unknown traffic via the ISP

Now that the default route exists on R1, we need to advertise it into OSPF so that other routers can learn it dynamically.
This is done using the default-information originate command under the OSPF process:

Figure 3 - Advertising the default static route in OSPF

In R1 we applied this command:

R1(config)# router ospf 1
R1(config-router)# default-information originate

This tells R1 to inject the default route into OSPF but only if the route exists in the routing table.

We can verify OSPF status with:

R1# show ip protocols
Routing Protocol is "ospf 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Router ID 203.0.115.1
  It is an autonomous system boundary router
  Redistributing External Routes from,
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
    192.168.1.0 0.0.0.255 area 0
    192.168.2.0 0.0.0.255 area 0
  Routing Information Sources:
    Gateway         Distance      Last Update
    203.0.115.1     110           00:00:03
  Distance: (default is 110)

This confirms: