How to Configure OSPF Default Route on a Cisco Router

In any network, a default route serves as a fallback option for routers to forward traffic if there is no specific path to the destination of the traffic in the routing table.

When a router receives a packet, it searches its routing table for a route to the destination. If a route exists, the router forwards the traffic through the shortest available path. However, if no route is found, the router resorts to the default route. If there’s neither a specific route nor a default route configured, the router discards the traffic.

Typically, the default route points towards the internet since routers cannot have routes to every possible destination in the internet. Therefore, any traffic without a matching route in the routing table is presumed to be destined for the internet and is forwarded accordingly using the default route.

In networks employing static routes, configuring a default route involves using the “ip route 0.0.0.0 0.0.0.0 next-hop address” command from the router’s privileged exec mode. However, with OSPF and other dynamic routing protocols, the “default-information originate” command facilitates the injection of default routes into the routing protocol, enabling their propagation throughout the network.

In this post, I will demonstrate how to configure an OSPF default route using a sample network.

Network Topology

Our network consists of two routers, R1 and R2. R1 is directly connected to R2 via Ethernet interfaces. We’ll configure default route on R2, which will then be propagated to R1.

How to Configure OSPF Default Route on a Cisco Router

Here are the steps to configure OSPF default route for the network shown above:

Step 1: Configure The interfaces of the Routers

Router 1

R1(config)#interface ethernet0/0
R1(config-if)#ip address 192.168.12.1 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#exit

Router 2

R2(config)#interface ethernet0/0
R2(config-if)#ip address 192.168.12.2 255.255.255.0
R2(config-if)#no shutdown 
R2(config-if)#exit

R2(config)#interface ethernet0/1
R2(config-if)#ip address 10.254.4.70 255.255.255.0
R2(config-if)#no shutdown 
R2(config-if)#exit

Step 2: Configure static default route

Because it is router 2 that has a connection to the internet, we will configure the default route on it.

R2(config)#ip route 0.0.0.0 0.0.0.0 10.254.4.254

 

Step 3: Configure OSPF on the routers

Router 1

R1(config)#router ospf 1
R1(config-router)#router-id 1.1.1.1
R1(config-router)#network 192.168.12.0 0.0.0.255 area 0
R1(config-router)#exit

 Router 2

R2(config)#router ospf 1
R2(config-router)#router-id 2.2.2.2
R2(config-router)#network 192.168.12.0 0.0.0.255 area 0
R2(config-router)#exit

Step 4: Configure OSPF Default route

After configuring the static route as we did in step 2, the default route does not propagate to all other OSPF routers. To enable this, we need to use the “default-information originate” command on the router for which the static default route is configured.

R2(config)#router ospf 1
R2(config-router)#default-information originate always 
R2(config-router)#exit

Step 5: Verify the Configuration

You can verify that the default route has been created by using the “show ip route command” as follow;

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
       i - IS-IS, su - IS-IS summary, 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, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is 192.168.12.2 to network 0.0.0.0

O*E2  0.0.0.0/0 [110/1] via 192.168.12.2, 00:01:27, Ethernet0/0<--OSPF Default Route
      192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.12.0/24 is directly connected, Ethernet0/0
L        192.168.12.1/32 is directly connected, Ethernet0/0

Related Content;

1. Configuring OSPF Hello Timer
2. How to Configure OSPF DR and BDR
3. How to Configure OSPF Passive Interface on Cisco Router
4. How to Configure OSPF Backbone Area | How to Configure OSPF Area 0
5. How to Configure OSPF Multi-Area
6. How To Configure OSPF on Layer 3 Switch In Packet Tracer
7. Redistributing Static Routes into OSPF: Explained With Example

Reference: https://github.com/misterkrittin/CCNA-Labs