OSPF Metric Cost

OSPF uses the OSPF cost in order to calculate the best path to the destination. This cost comes from the speed your interfaces are operating at.
As you can see below, we have different types of interfaces, each with a cost number assigned.

Figure 1 – OSPF cost per interface

To check the OSPF cost on our interfaces, we can use show ip ospf interface brief

R1# show ip ospf interface brief
Interface             PID   Area    IP Address/Mask            Cost  State  Nbrs
FastEthernet0/0         1   0     192.168.10.1/255.255.255.0   1     DR     0/0
GigabitEthernet0/0      1   0     192.168.12.1/255.255.255.0   1     DR     0/0
GigabitEthernet0/1      1   0     192.168.13.1/255.255.255.0   1     BDR    0/0
Ethernet0/0             1   0     192.168.11.1/255.255.255.0   100   DR     0/0

Here, we can clearly see that:

• FastEthernet and GigabitEthernet interfaces have a cost of 1

• Ethernet (10 Mbps) has a cost of 100

OSPF uses these values to evaluate the efficiency of each OSPF link.
The lower the cost, the more preferred the path.

In a typical OSPF network, to determine which path can be used to send packets to the destination, OSPF accumulates the cost of all the outgoing interfaces along the way and chooses the path with the lowest total cost.

Figure 2 – OSPF Topology

In this example, we can check the route selected for 192.168.20.0/24 using:

R1# show ip route 192.168.20.0
Routing entry for 192.168.20.0/24
  Known via "ospf 1", distance 110, metric 3, type intra area
  Last update from 192.168.4.2 on GigabitEthernet0/1, 00:00:10 ago
  Routing Descriptor Blocks:
  * 192.168.4.2, from 192.168.20.1, 00:00:10 ago, via GigabitEthernet0/1
      Route metric is 3, traffic share count is 1

As you can see, the OSPF cost is 3 when using the path:
R1 -> R5 -> R4 -> 192.168.20.0/24
Cost: 1 + 1 + 1 = total cost 3

The alternative path is:
R1 -> R2 -> R3 -> R4 -> 192.168.20.0/24
Cost: 1 + 1 + 1 + 1 = total cost 4

Figure 3 – OSPF chooses the path with the lowest total cost

OSPF selects the first path because it has the lowest metric.

The OSPF cost is calculated using the following formula:

Figure 4 – Formula used to calculate OSPF cost

The reference bandwidth (in Mbps) is, by default, set to 100 Mbps.
The link bandwidth is determined by your interface type on your OSPF router.

Imagine you have a FastEthernet 100 Mbps interface:
→ The calculation is: Cost = 100 / 100 = 1

Pretty easy, right?

This default reference value was defined a long time ago, back when 100 Mbps was considered fast.
But today, network speeds have evolved a lot and that creates a problem.

Let’s look at the different interface types, their bandwidths, and their default OSPF costs:

OSPF Default Cost per Interface Type

Table 1 – Default OSPF cost values per interface type (based on 100 Mbps reference).

If you examine the table above, you'll notice a key detail: from 100 Mbps and more, the OSPF cost stays fixed at 1, even as interface speeds increase. That’s because the cost is calculated using the formula 100 / bandwidth, and OSPF cost only works with whole numbers.

When the result is a decimal, like 0.1 for a 1 Gbps link, it gets rounded up to 1. This means that whether you're using FastEthernet, Gigabit, or even 10 Gbps interfaces, OSPF sees them all as equal in cost.

The Problem With Default Cost

In this OSPF topology, all links with a bandwidth of 100 Mbps or more are assigned the same cost: 1.

Figure 5 – OSPF sees 100 Mbps and 1 Gbps as equal