![R1's show ip route output lists connected and local routes for 10.1.1.0/30, 10.1.1.1/32, 10.1.1.101/32, 192.168.1.0/24 and 192.168.1.1/32, plus an OSPF route O 172.16.1.0/24 [110/2] via 10.1.1.2, GigabitEthernet0/1.](https://assets.achievable.me/productFiles/c3e1cca4-451f-40e9-9474-0eaf23f814fe/39308-show-ip-route-command-on-r1--20260930220648--2ccbf56b7fe39d014e8fbd2c7d6c320dfc36b73db9456d0d6bb5c96be07f6cad.webp)
![R2 show ip route output with code legend: connected routes 10.1.1.0/30 and 172.16.1.0/24 on Gig0/1 and Gig0/0, and an OSPF route O 192.168.1.0/24 [110/2] via 10.1.1.1, learned through Gig0/1.](https://assets.achievable.me/productFiles/c3e1cca4-451f-40e9-9474-0eaf23f814fe/39309-show-ip-route-command-on-r2--20260930220649--5164c4037ae3072e19f36cfbed6d73ad2270677e0394bc1c9e721b664da64542.webp)
How to configure OSPFv2 on Cisco routers
Loopback interfaces
Before configuring OSPF, you need to understand a special type of interface called a loopback interface.
A loopback is a logical interface. It doesn’t correspond to a physical port, but it can still be assigned an IP address.
- On end hosts (Windows, macOS, Linux), the loopback IP address is 127.0.0.1.
- The main purpose on PCs and servers is to verify that the operating system’s TCP/IP stack is working correctly.
When you ping 127.0.0.1 or run application tests against it, the traffic stays internal to the device. You can think of it as the host “talking to itself” to confirm it can process the layers of the TCP/IP model.
Routers can also have loopback interfaces, but they’re typically used to verify connectivity between routers. The key advantage is stability:
- A loopback interface stays up as long as the router is up.
- A physical interface can go down if a cable is disconnected or a link fails.
So, if one physical link fails between two routers, you can often still reach the remote router’s loopback interface (as long as you have redundant links and a valid route to that loopback IP).
Unlike end hosts, routers don’t have loopback interfaces by default. You must create them, and you can create multiple loopbacks on the same router.
To create a loopback interface on a router, go to Global Configuration mode and enter:
Network administrators often use Loopback0 as the first loopback interface, but the number is arbitrary. It’s only used to identify the loopback interface.
After you create the loopback interface, it will automatically be in the up state (unlike many physical interfaces). While still in Interface Configuration mode, assign it an IP address.
For the subnet mask, you typically use 255.255.255.255 for a loopback interface. That’s a /32 host address, and it doesn’t represent an entire subnet.
If you haven’t manually configured a routing protocol Router-ID, the IP address on a loopback interface is also used by the router as the routing protocol Router-ID.
Configuring OSPFv2
Suppose we want to configure OSPF between two routers connected over a WAN, and each router has a LAN attached.
- R1 is connected to the 192.168.1.0/24 network.
- R2 is connected to the 172.16.1.0/24 network.
- The link between the routers uses the 10.1.1.0/30 network.
R1 and R2 want to advertise their directly connected LANs to each other using OSPF.
First, enter Global Configuration mode and enable the OSPF process. We’ll use a Process ID of 1.
The OSPF Process ID is arbitrary and locally significant. It is not advertised to the other router.
This puts you into Routing Protocol Configuration mode, and the prompt changes to:
OSPF router-ID selection
After enabling OSPF, the router must have an OSPF Router-ID. There are three ways the Router-ID can be selected.
A Router-ID is a 32-bit value written in dotted-decimal notation. It doesn’t have to be an IP address that actually exists on the router.
You can manually configure the OSPF Router-ID under the router ospf configuration mode using:
Here, “x.x.x.x” is the Router-ID.
However, if you don’t manually specify the Router-ID, then the router will use a Loopback interface with the highest IPv4 address out of all loopback interfaces. If no loopback interfaces exist, it will select the router interface with the highest IPv4 address out of all router interfaces.
Without the Router-ID, the OSPF process on the router cannot function.
Influencing the OSPF DR/BDR selection
In Chapter 3.7 OSPF neighbor adjacencies, we discussed that the OSPF interface priority is the first method used to determine which router becomes the DR during the DR/BDR election (part of the OSPF broadcast adjacency process).
- The router with the highest OSPF interface priority becomes the DR.
- The router with the second-highest interface priority becomes the BDR.
To configure the OSPF interface priority, enter Interface Configuration mode on the interface that will form the OSPF adjacency, then use:
Example: set the OSPF interface priority to 255 on GigabitEthernet0/0 so it always becomes the DR on that segment:
If the OSPF priority ties between routers, the router with the highest Router-ID becomes the DR. The router with the second-highest Router-ID becomes the BDR, and it takes over as DR only if the current DR fails.
Next, we need to form the adjacency between the two routers by enabling OSPF on the router interfaces (see Chapter 3.7 OSPF neighbor adjacencies). To enable OSPF on interfaces, we configure network statements.
Network statements and wildcard masks
A network statement tells the routing protocol which router interfaces should participate in the routing process.
Here’s what happens when a router interface matches a network statement in OSPF:
- OSPF is enabled on that interface.
- The router starts sending OSPF Hellos out that interface.
- The router creates LSAs for the directly connected network on that interface.
- After an adjacency forms, those LSAs can be advertised to the OSPF neighbor.
OSPF network statements are structured like this:
| Command | Network of the router interface | Wildcard Mask | OSPF Area ID |
|---|
To configure a network statement, you also need to understand the wildcard mask.
A wildcard mask is the inverse of a subnet mask. It defines the range of IP addresses that an interface can fall within to match the network statement.
To derive a wildcard mask from a subnet mask, subtract the subnet mask from 255.255.255.255.
Example #1: wildcard mask for subnet mask 255.255.255.0 (/24):
The wildcard mask is 0.0.0.255.
Example #2: wildcard mask for subnet mask 255.255.255.252 (/30):
The wildcard mask is 0.0.0.3.
Now we can enable OSPF on the two router interfaces on R1 by entering these commands under OSPF Routing Protocol Configuration mode:
| Command | Network of the router interface | Wildcard Mask | OSPF Area ID |
|---|---|---|---|
| network | 192.168.1.0 | 0.0.0.255 | area 0 |
| network | 10.1.1.0 | 0.0.0.3 | area 0 |
That is:
We must also do the same thing on R2:
| Command | Network of the router interface | Wildcard Mask | OSPF Area ID |
|---|---|---|---|
| network | 172.16.1.0 | 0.0.0.255 | area 0 |
| network | 10.1.1.0 | 0.0.0.3 | area 0 |
After you enter the network statements on both routers, they exchange Hellos on the shared link and go through the OSPF neighbor adjacency process (see Chapter 3.7 OSPF neighbor adjacencies).
You’ll see a console message indicating the neighbor state changed from “LOADING to FULL”. That indicates the adjacency is up.
To view established adjacencies, use this command in Privileged EXEC mode:
Now check whether R1 learned any OSPF routes:
R1 has learned the route to the 172.16.1.0/24 network from R2.
On R2, you should see the 192.168.1.0/24 network learned from R1:
To view how a router interface is configured for OSPF, use:
This command shows details such as:
- The IP address of the interface running OSPF
- The area the interface is in
- The OSPF Process ID
- The Router-ID
- The adjacency type (Network Type - BROADCAST vs. POINT-TO-POINT)
- The interface cost
- The interface priority (default is 1)
- Which router is the DR and which is the BDR
- The Hello and Dead timers
Which method did R1 use to generate the Router-ID?
It used the router interface with the highest IPv4 address
Which OSPF adjacency type (Network Type) is being used on the GigabitEthernet0/1 router interface on R1?
BROADCAST
Who is the DR in this adjacency and why?
R1, because it has a higher Router-ID
Is R1’s GigabitEthernet0/0 and GigabitEthernet0/1 router interfaces using the default OSPF Hello and Dead timers for their adjacency type?
Yes, this is the default Hello and Dead timers for Broadcast adjacencies.
If these criteria aren’t met (or if values that must match don’t match), the OSPF adjacency between the two routers will not come up.