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).

(Image 181)

Image Title - High availability after a link failure, due to a Loopback interface on R2

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:

interface loopback [loopback #]

Network administrators often use Loopback0 as the first loopback interface, but the number is arbitrary. It’s only used to identify the loopback interface.

(Image 182)

Image Title - Creating a Loopback interface on R1

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.

(Image 183)

Image Title - Assigning an IP address to the Loopback0 interface on R1

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.

(Image 184)

Image Title - R1 and R2 with their respective networks.

• 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.

router ospf 1

This puts you into Routing Protocol Configuration mode, and the prompt changes to:

R1(config-router)#

(Image 185)

Image Title - Configuring the OSPF process on R1

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:

router-id x.x.x.x

Here, “x.x.x.x” is the Router-ID.

For example, to configure a Router-ID of “10.1.1.1” on R1:

R1(config)#configure terminal R1(config-router)#router ospf 1 R1(config-router)#router-id 10.1.1.1

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.

The Router-ID selection process can be summarized like this, in order of precedence:

1. Select the Router-ID that was manually configured (router-id [x.x.x.x]).
2. If a Router-ID is not manually configured, then select the Loopback interface with the highest IPv4 address out of all the Loopback interfaces on the Router.
3. If there are no Loopback interfaces on the Router, select the router interface with the highest IPv4 address out of all the router interfaces configured with IPv4 addresses.

Without the Router-ID, the OSPF process on the router cannot function.

Influencing the OSPF DR/BDR selection

In Chapter 3.5 OSPFv2, 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:

ip ospf priority [0-255]

The OSPF Priority value ranges from 0-255. Setting the OSPF Priority value to 255 using the “ip ospf priority 255” command underneath a router interface ensures that the Router will be the DR for any OSPF adjacency that it forms on that router interface, because it’s the highest value.

Setting the OSPF Priority value to 0 using the “ip ospf priority 0” command disables the router interface from participating in a DR/BDR election, meaning, it would never become the DR or BDR for any OSPF adjacencies that it would form on that router interface.

Example: set the OSPF interface priority to 255 on GigabitEthernet0/0 so it always becomes the DR on that segment:

interface gigabitethernet0/0 ip ospf priority 255

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 3.5 OSPFv2). 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: wildcard mask for subnet mask 255.255.255.0 (/24):

$$\mathrm{255.255.255.255}$$

$$\frac{-\mathrm{255.255.255.0}}{\mathrm{0.0.0.255}}$$

(Image 186)

Image Title - Calculating the Wildcard Mask for a Subnet Mask of 255.255.255.0

The wildcard mask is 0.0.0.255.

Example: wildcard mask for subnet mask 255.255.255.252 (/30):

(Image 187)

Image Title - Calculating the Wildcard Mask for a Subnet Mask of 255.255.255.252

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:

router ospf 1 network 192.168.1.0 0.0.0.255 area 0 network 10.1.1.0 0.0.0.3 area 0 end

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

(Image 188)

Image Title - Configuring the OSPF on R2

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 3.5 OSPFv2).

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:

show ip ospf neighbor

(Image 189)

Image Title - show ip ospf neighbor command on R1

Now check whether R1 learned any OSPF routes:

(Image 190)

Image Title - show ip route command on R1

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:

(Image 191)

Image Title - show ip route command on R2

To view how a router interface is configured for OSPF, use:

show ip ospf interface

(Image 192)

Image Title - show ip ospf interface on R1

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?

Which OSPF adjacency type (Network Type) is being used on the GigabitEthernet0/1 router interface on R1?

Who is the DR in this adjacency and why?

Is R1’s GigabitEthernet0/0 and GigabitEthernet0/1 router interfaces using the default OSPF Hello and Dead timers for their adjacency type?

One final thing that you must know about forming OSPF adjacencies which is crucial to troubleshooting OSPF and also for the CCNA 200-301 v1.1 exam, is the criteria that must be met in order for two Routers to form an adjacency:

• The link that the two Routers are using to form the adjacency must be in the same Area on both Routers.
• The link that the two Routers are using to form the adjacency must be in the same IP network.
• The Hello and Dead timers must match
• The Router-ID on both Routers must be unique. In other words, they cannot be the same.

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.

Also, remember that the Process ID is only locally significant therefore, you can use the same Process ID on both Routers.