Networking Basics: OSPF Protocol Explained

Open Shortest Path First (OSPF) is a standard routing protocol that’s been used the world over for many years. Supported by practically every routing vendor, as well as the open source community, OSPF is one of the few protocols in the IT industry you can count on being available just about anywhere you might need it.

Enterprise networks that outgrow a single site will often use OSPF to interconnect their campuses and wide area networks (WANs).

If you’re considering a dynamic routing protocol because your network has outgrown static routes, OSPF might seem a little daunting. It’s not quite as easy to set up as EIGRP so the temptation might be to simply use EIGRP and avoid the intimidating terminology that comes along with a complete understanding of OSPF.

My recommendation is not to let OSPF scare you. It’s true that OSPF in large implementations can be complex. However, an OSPF configuration supporting smaller networks can be comparatively simple.

In this post, I’ll discuss some of OSPF’s major principles, and then follow up with a simple configuration that brings up OSPF between two Cisco routers and exchange routes.

OSPF’s big idea

OSPF is a routing protocol. Two routers speaking OSPF to each other exchange information about the routes they know about and the cost for them to get there.

When many OSPF routers are part of the same network, information about all of the routes in a network is learned by all of the OSPF routers within that network—technically called an area. (We’ll talk more about the area as we go on).

Each OSPF router passes along information about the routes and costs they’ve heard about to all of their adjacent OSPF routers, called neighbors.

OSPF routers rely on the cost to compute the shortest path through the network between themselves and a remote router or network destination. The shortest path computation is done using Dijkstra’s algorithm. This algorithm isn’t unique to OSPF. Rather, it’s a mathematical algorithm that happens to have an obvious application to networking.

Consider a simple example of five routers connected as shown in the diagram below. Assuming all links have the same cost, what’s the fastest way for R3 to connect to R5? Through R4 — R4 is the lowest cost path. (R3’s path to R5 via R1, for example, adds another link and therefore additional cost.)

OSPF interfaces

Another important idea in OSPF is that interfaces used to exchange information with OSPF neighbors have different types. There are too many types to discuss here but you should be aware of two important ones.

1. An OSPF broadcast interface is connected to a shared network, like Ethernet.
2. An OSPF point-to-point interface is connected to a link where there can only be a single OSPF router on either end, such as a WAN link or a purpose-built Ethernet link.

The reason for the various interface types is to make sure that all routers know about all routes from all other routers.

On point-to-point links, there’s no mystery — the two routers know they’re the only OSPF routers on the link and so they exchange routes with each other.

On broadcast links, there’s a potential for many different OSPF routers to be on the network segment. To minimize the number of neighbor relationships that form on broadcast links, OSPF elects a designated router (as well as a backup) whose job it is to neighbor with all other OSPF routers on the segment and share everyone’s routes with everyone else.

OSPF areas

Areas in OSPF are collections of routers grouped together. With the exception of area border routers, OSPF routers in one area don’t neighbor with routers in other areas. Among other reasons, areas were once used to scale large OSPF networks.

Back when router CPUs were less powerful than they are today, a general rule of thumb was to keep an OSPF area to no more than 50 routers. That would keep the number of OSPF shortest path computations and database updates to a manageable amount as interfaces went up and down, routes were learned and withdrawn, and so on.

In modern networks, it’s not unheard of to scale to a thousand routers or more in a single area — router CPUs have come a long way.

Today, although the scale is not much of a reason for implementing multiple areas, OSPF areas are still useful as administrative boundaries in a network. For example:

• Route summarization & aggregation (replacing several small routes with one larger route that covers them) can only happen at OSPF area boundaries.
• Not all routers need to know about every other route available in a network. Using OSPF areas, it’s possible to inject a default route representing all routes outside of the local area.

If you’re thinking you should be able to summarize or filter routes between OSPF routers within an area, the problem is that for the shortest path first (SPF) calculation to work, all routers in an area need to have an identical “picture” of the network. Therefore, it’s impossible to hide routes between OSPF routers in an area.

(A type of OSPF filtering you might be familiar with is actually a filter between the OSPF routing information base (RIB) and the router’s forwarding information base (FIB). The local OSPF process still passes information about the filtered route along to other OSPF neighbors.)

The most important area in OSPF is the backbone area, also known as area 0. The backbone area is the area that all OSPF areas must traverse to get to other OSPF areas.

For instance, let’s say we have area 0, area 1, and area 2. Area 1 traffic must traverse Area 0 to get to Area 2, and vice versa. Even if there was a router with one interface in Area 1, and another interface in Area 2, area 1 and 2 traffic could not traverse this router directly. The reason for this is loop prevention.

While OSPF routers within an area know everything there is to know about the network topology, topology information is hidden at area borders. For more detail about why the backbone area and related traversal rule exists, network architect Jeff Doyle has an article that explains it well.

A simple two-router OSPF network

Here’s an example of a network configuration that creates a very simple OSPF network between two Cisco routers. The routers are placed in area 0 and an OSPF point-to-point link is configured between them. R1 will announce the 1.1.1.1/32 route and R2 will announce 2.2.2.2/32. Let’s walk through it.

R1’s configuration

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface GigabitEthernet2
 description OSPF Transit
 ip address 10.200.1.1 255.255.255.252
! Set this interface type to be “point to point”. Put another way, there will be no “designated router” on this segment.
 ip ospf network point-to-point
!
! Start an OSPF process with an ID of 200. This process number isn’t super important -- both sides do *not* have to match. The process ID distinguishes it from other OSPF processes you might be running on your router.
router ospf 200
! If you don’t set an OSPF router ID, the router will choose the highest IP of a loopback interface. If there are no loopback interfaces, the highest IP assigned to any router interface will be chosen. If you change the router ID, you’ll have to clear the OSPF process for it to become active, briefly knocking down OSPF adjacencies with neighbors.
 router-id 10.200.2.1
! As OSPF neighbors communicate, details are logged. Can be helpful in troubleshooting OSPF adjacency issues.
 log-adjacency-changes detail
! Place any interface with an IP address matching 1.1.1.1/32 into OSPF area 0. This line will match interface Loopback0 on this router.
 network 1.1.1.1 0.0.0.0 area 0
! Place any interface with an IP address matching 10.200.1.0/30 into OSPF area 0. This line will match interface GigabitEthernet2 on this router.
 network 10.200.1.0 0.0.0.3 area 0

R2’s configuration

interface Loopback0
 ip address 2.2.2.2 255.255.255.255
!
interface GigabitEthernet2
 description OSPF Transit
 ip address 10.200.1.2 255.255.255.252
 ip ospf network point-to-point
!
router ospf 200
 router-id 10.200.1.2
 log-adjacency-changes detail
 network 2.2.2.2 0.0.0.0 area 0
 network 10.200.1.0 0.0.0.3 area 0

Basic OSPF commands

Now that OSPF is configured, let’s look at a few basic OSPF commands available on Cisco IOS and similar industry-standard CLIs. I’ve truncated some of the command output to make it easier to read and explain.

The command “show ip ospf neighbor” displays OSPF neighbors and their state. In this case, we see R1 and R2 fully adjacent to each other via their GigabitEthernet 2 interfaces.

The neighbor ID equals the router ID of the neighbor.

The priority is related to the election of a designated router — not important for our simple example.

On a point-to-point link, the OSPF state should be “full.” If it’s not, something has probably gone wrong.

The dead time is a countdown timer constantly being reset as messages are heard from the neighbor. If the dead time gets to zero, the neighbor is presumed dead, the adjacency is torn down, and the link is removed from SPF calculations in the OSPF database.

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.200.1.2        0   FULL/  -        00:00:33    10.200.1.2      GigabitEthernet2
R1#

R2#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.200.2.1        0   FULL/  -        00:00:30    10.200.1.1      GigabitEthernet2
R2#

When looking at a device’s forwarding table, the “show ip route ospf” shows just the routes that have entered the forwarding table via OSPF.

In our case, R1 will know the 2.2.2.2/32 route via OSPF, while R2 will know 1.1.1.1/32 via OSPF. Why doesn’t the 10.200.1.0/30 route show up as an OSPF route on either R1 or R2? Because 10.200.1.0/30 is also a connected route. Connected routes trump OSPF-learned routes.

R1#show ip route ospf
      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/2] via 10.200.1.2, 00:43:45, GigabitEthernet2
R1#

R2#show ip route ospf
      1.0.0.0/32 is subnetted, 1 subnets
O        1.1.1.1 [110/2] via 10.200.1.1, 00:44:13, GigabitEthernet2
R2#

Helpful OSPF guides

There’s a lot more information about OSPF than we can explore in a blog post. In fact, entire books have been written on the topic. My hope is that your curiosity has been piqued to go exploring and learn more.

Over my years of operating OSPF networks, there are certain references I’ve referred to repeatedly. I hope they help you as much as they’ve helped me.

• Open Shortest Path First (Wikipedia)
• OSPF Design Guide (Cisco)
• OSPF Neighbor States (Cisco)
• OSPF Neighbor Problems Explained (Cisco)
• Troubleshooting OSPF (Cisco)