OSPF Metric – Cost – Saikat Infotech Blog

OSPF Metric - Cost

This lesson discusses how OSPF determines the best paths to destinations using its metric called “OSPF Cost”. It is a fundamental concept of the operation of the link-state routing protocol that is important for CCNA candidates to understand well.

What is OSPF Cost?

Every dynamic routing protocol, such as OSPF, has a way to compare different paths to a destination and determine which one is best. Every protocol uses different methods of comparison based on different metric systems. OSPF uses a metric called OSPF Cost based on each hop’s outgoing interface bandwidth.

Every router calculates the Cost of every OSPF-enabled link using a formula we will see later. The cost is a property of every link, as shown in the diagram below.

				
					R1# sh ip ospf database router 1.1.1.1
            OSPF Router with ID (1.1.1.1) (Process ID 1)
                Router Link States (Area 0)
  LS age: 277
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 1.1.1.1
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000021
  Checksum: 0xB6A0
  Length: 72
  Number of Links: 4
    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 10.6.7.0
     (Link Data) Network Mask: 255.255.255.0
      Number of MTID metrics: 0
       TOS 0 Metrics: 1
    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 10.3.1.0
     (Link Data) Network Mask: 255.255.255.0
      Number of MTID metrics: 0
       TOS 0 Metrics: 10
    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.1.0.1
     (Link Data) Router Interface address: 10.1.0.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 1
    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.0.5.1
     (Link Data) Router Interface address: 10.0.5.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 10

				
					R1# sh ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Lo0          1     0               172.16.1.1/24      1     P2P   0/0
Gi0/0        1     0               10.1.3.1/24        1     DR    1/1
Fa1/0        1     0               10.1.2.1/24        1     P2P   0/0
Et0/1        1     0               10.1.1.1/24        10    DR    1/1
Et0/2        1     0               10.1.1.1/24        10    P2P   0/0

By default, the Reference Bandwidth is 100 Mbps. The Link Bandwidth is the actual bandwidth of the interface (in bits per second). Using the default Reference Bandwidth, we can calculate the cost of the following interface types:

Table 1. Cost of different interface types.
Interface	Formula [Mbps]	Cost
DSL (768 Kbps)	100/0.768	133
DS1 (1.544 Mbps)	100/1.544	64
Ethernet (10 Mbps)	100/10	10
FastEthernet (100 Mbps)	100/100	1
GigabitEthernet (1 Gbps)	100/1000	1
TenGigabitEthernet (10 Gbps)	100/10000	1
40-GigabitEthernet (40 Gbps)	100/40000	1
100-GigabitEthernet (100 Gbps)	100/100000	1

Notice that when the interface bandwidth is greater than the reference bandwidth, the value is rounded to 1. For example, in the case of a 1Gbps interface – the cost is calculated as 100/1000, which is 0.1, but it is rounded to Cost 1.

From the table, it is obvious that every high-speed interface has the same Cost of 1. That’s because the default Reference Bandwidth value of 100Mbps was set back in a time when interface speeds rarely surpassed 10 Mbps. However, even 100Mbps is considered slow by today’s network standards.

How does OSPF Cost work?

OSPF calculates the sum of the costs for all outgoing interfaces in a path to a destination. Then, it compares the sum of each available path to the destination and chooses the route with the lowest total cost.

Let’s take the following diagram as an example. There are three possible paths from R1 to subnet 172.16.8.0/24.

The following table compares the sum of the cost of the outgoing interfaces of each path.

Table 1. The total cost of each available path is in Figure 3.
Route	Total Cost [lower is better]
R1-R2-R3-R4-R5-R8	50
R1-R5-R6-R8	40
R1-R7-R8	30 (best)

As a result, R1 adds the route via R7 to its routing table as best for destination 172.16.8.0/24.

				
					R1# sh ip route 172.16.8.0
Routing entry for 172.16.8.0/24
  Known via "ospf 1", distance 110, metric 30, type intra area
  Last update from 10.1.3.2 on Ethernet0/2, 00:00:03 ago
  Routing Descriptor Blocks:
  * 10.1.3.2, from 8.8.8.8, 00:00:03 ago, via Ethernet0/2
      Route metric is 30, traffic share count is 1

Let’s compare the three available routes to 172.16.8.0/24 in the context of the link state protocol.

Table 2. The total cost of each available path is in Figure 4.
Path	Total Cost [lower is better]
R1-R2-R3-R4-R5-R8	5
R1-R5-R6-R8	4
R1-R7-R8	3 (best)

According to the default OSPF best-path algorithm, the path R1-R7-R8 is the best to 172.16.8.0/24. However, is it really?

If you look at the diagram again, you can easily figure out that you can send 40 Gbps over the path via R2-R3-R4-R8. Also, you can transmit 10 Gbps via R5-R6-R8. However, over the path via R7-R8, you can only transfer 1Gbps, so the path that OSPF has chosen as best is actually the slowest of the three.

The problem is that because of the default reference bandwidth of 100Mbps, the calculated cost of every link is 1. Fortunately, the protocol was designed to account for that and allow us to change the reference bandwidth. Let’s see how.

Manipulating the OSPF Cost

Since the metric formula has three components, there are three ways to manipulate the OSPF cost of links, as shown in the diagram below.

				
					R1# conf t
Enter configuration commands, one per line.  End with CNTL/Z
R1(config)# router ospf 1
R1(config-router)# auto-cost reference-bandwidth ?
  <1-4294967>  The reference bandwidth in terms of Mbits per second
R1(config-router)# auto-cost reference-bandwidth 40000
% OSPF: Reference bandwidth is changed.
        Please ensure reference bandwidth is consistent across all routers.
R1(config-router)# end
R1#

The result of this change is shown in the following table. The routing protocol can now differentiate that a 100Mbps link is slower than a 10 Gbps link, which leads to a more optimal best-path selection.

Table 3. Cost of different interface types when Ref Bandwidth is set to 40 Gbps.
Interface	Formula [Mbps]	Cost
DSL (768 Kbps)	40000/0.768	52084
DS1 (1.544 Mbps)	40000/1.544	25907
Ethernet (10 Mbps)	40000/10	4000
FastEthernet (100 Mbps)	40000/100	400
GigabitEthernet (1 Gbps)	40000/1000	40
TenGigabitEthernet (10 Gbps)	40000/10000	4
40-GigabitEthernet (40 Gbps)	40000/40000	1
100-GigabitEthernet (100 Gbps)	40000/100000	1

Now, let’s see what happens when configuring this on every router in the topology (the change is performed per device).

				
					 We apply this to every router - R1 through R8
router ospf 1
 auto-cost reference-bandwidth 40000

Now, the routing protocol calculates the following values for each of the three available paths from R1 to 172.16.8.0/24.

Table 3. The total cost of each available path is in Figure 6.
Path	Total Cost [lower is better]
R1-R2-R3-R4-R5-R8	5 (best)
R1-R5-R6-R8	13
R1-R7-R8	81

You can see that now the best route is via R2-R3-R4-R8, which is indeed the fastest possible path to the destination because it allows end-to-end data transmission at a 40 Gbps rate.

There are two very important recommendations when it comes to the OSPF Reference Bandwidth:

Set the Reference Bandwidth to a value equal to the fastest link in the network.
- For example, if the fastest link in the domain is 40 Gbps, you set the Reference Bandwidth to 40 Gbps.
Ensure that all routers in the network are configured with the same auto-cost reference-bandwidth value.
- Since the auto-cost reference-bandwidth value is configured on each device separately, changing it across the entire network is susceptible to misconfiguration. For example, you can forget to reconfigure one of the devices, which stays with the default value of 100Mbps. This can lead to suboptimal routing. Always ensure that every device is configured with the same reference bandwidth.

Method 2. The interface subcommand

There is another more granular and direct way to manipulate the cost of a link. We can directly configure it under the interface configuration level, as shown in the output below:

				
					R1# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)# interface Ethernet0/0
R1(config-if)# ip ospf cost ?
  <1-65535>  Cost
R1(config-if)# ip ospf cost 50
R1(config)# interface Ethernet0/1
R1(config-if)# ip ospf cost 100
R1(config)# interface Ethernet0/2
R1(config-if)# ip ospf cost 200
R1(config-if)# end
R1#

You can see how we can directly configure the OSPF Cost of a link. The output of the sh ip ospf interface brief command shows the Cost values we configured under the interfaces.

				
					R1# sh ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Lo0          1     0               172.16.1.1/24      1     P2P   0/0
Et0/2        1     0               10.1.3.1/24        200   BDR   1/1
Et0/1        1     0               10.1.2.1/24        100   BDR   1/1
Et0/0        1     0               10.1.1.1/24        50    BDR   1/1

This method is typically used when you want to influence the OSPF traffic path for a particular interface. For example, an interface experiences some packet loss, and you want to move all traffic away from the interface. What do you do? You configure a very high OSPF cost on that link (for example, cost 1,500), and the SPF algorithm makes it less preferred and doesn’t use it as the best path to any destination.

Method 3. The interface bandwidth

The third method of manipulating the OSPF cost is to change the interface bandwidth. Each router interface has a default bandwidth value that represents the interface speed.
By default, the interface bandwidth value matches the physical transmission speed of the link, as shown in the example below. For example, the bandwidth of the Ethernet interface is set to 10Mbps. The bandwidth of the FastEthernet interface is set to 100Mbps. The bandwidth of the GigabitEthernet interface is set to 1000Mbps and so on.

				
					R1# show interfaces Eth0/0
Ethernet0/0 is up, line protocol is up
  Hardware is AmdP2, address is aabb.cc00.1000 (bia aabb.cc00.1000)
  Internet address is 10.1.1.1/24
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:01, output 00:00:03, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     14144 packets input, 2194919 bytes, 0 no buffer
     Received 14115 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 input packets with dribble condition detected
     25123 packets output, 2863273 bytes, 0 underruns
     Output 14198 broadcasts (0 IP multicasts)
     0 output errors, 0 collisions, 2 interface resets
     7 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

Notice something very important: The interface bandwidth does not influence the interface’s actual transmission speed and does not necessarily need to match the transmission speed. For example, we can change the interface bandwidth of R1’s Ethernet0/0 interface to 40,000Mbps even though the interface cannot physically transmit at that speed.

The interface bandwidth has only a representative purpose.

Let’s reconfigure the interface bandwidth of R1’s Eth0/0 to 40,000Mbps.

				
					R1# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)# interface Eth0/0
R1(config-if)# bandwidth ?
  <1-200000000>  Bandwidth in kilobits
  inherit        Specify how bandwidth is inherited
  qos-reference  Reference bandwidth for QOS
  receive        Specify receive-side bandwidth
R1(config-if)# bandwidth 40000000
R1(config-if)# end
R1#

Now, if we check the interface bandwidth, we can see it is not set to the physical interface speed of 10Mbps but to the user-defined value of 40,000Mbps.

				
					R1# sh interface Eth0/0
Ethernet0/0 is up, line protocol is up
  Hardware is AmdP2, address is aabb.cc00.1000 (bia aabb.cc00.1000)
  Internet address is 10.1.1.1/24
  MTU 1500 bytes, BW 40000000 Kbit/sec, DLY 1000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     14953 packets input, 2318360 bytes, 0 no buffer
     Received 14924 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 input packets with dribble condition detected
     26563 packets output, 3025605 bytes, 0 underruns
     Output 15021 broadcasts (0 IP multicasts)
     0 output errors, 0 collisions, 2 interface resets
     7 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

Now, the OSPF cost of the interface has changed from 40000/10 (Cost 4000) to 40000/40000, which is equal to Cost 1.

Notice that other IOS features use the interface bandwidth to make decisions. It is not a good practice to change the default interface bandwidth unless really necessary. If you want to manipulate the OSPF best path decision, It is recommended to use the Reference Bandwidth value or the method of setting the OSPF cost directly on the interface using the ip ospf cost command.

OSPF Cost (Open Shortest Path First)

1. What is OSPF Cost?

OSPF cost is a metric used by the routing protocol OSPF (Open Shortest Path First) to determine the best path to reach a destination network.
In simple words, cost = path preference value.
The lower the cost, the better the path.

2. Why OSPF Uses Cost?

OSPF may have multiple paths to reach the same destination. To select the best one, it compares the total cost of each path.
The path with the lowest cumulative cost is chosen as the best route.

3. How OSPF Cost is Calculated?

OSPF cost is based on bandwidth using the formula:

$Bandwidth\textbf{Cost = Reference Bandwidth / Interface Bandwidth}$

👉 Default Reference Bandwidth = 100 Mbps (100,000,000 bps)

4. Common OSPF Cost Values (Default)

Interface Speed	Cost Calculation	OSPF Cost
10 Mbps	100 / 10	10
100 Mbps	100 / 100	1
1 Gbps	100 / 1000	1 (rounded)

⚠️ Note: For high-speed links (like 1 Gbps or more), cost becomes 1 unless reference bandwidth is changed.

5. Manual Cost Configuration (Cisco)

You can manually change OSPF cost in **Cisco routers:

Command:

				
					interface GigabitEthernet0/0
ip ospf cost 20

👉 This overrides automatic calculation.

6. Changing Reference Bandwidth (Important in Real Networks)

To support modern high-speed networks:

				
					router ospf 1
auto-cost reference-bandwidth 1000

👉 Now:

1 Gbps link → cost = 1
100 Mbps → cost = 10

Key Points to Remember

OSPF prefers lowest cost path
Cost is inversely proportional to bandwidth
Total path cost = sum of all interface costs
Default reference bandwidth = 100 Mbps
Can be manually configured

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OSPF Metric - Cost

What is OSPF Cost?

Method 3. The interface bandwidth

OSPF Cost (Open Shortest Path First)

1. What is OSPF Cost?

2. Why OSPF Uses Cost?

3. How OSPF Cost is Calculated?

$Bandwidth\textbf{Cost = Reference Bandwidth / Interface Bandwidth}$

4. Common OSPF Cost Values (Default)

5. Manual Cost Configuration (Cisco)

6. Changing Reference Bandwidth (Important in Real Networks)

Key Points to Remember

furniture

lightings

accessories

Texture lab

what’s new

what’s new

Flash sales

Flash sales

OSPF Metric - Cost

What is OSPF Cost?

Method 3. The interface bandwidth

OSPF Cost (Open Shortest Path First)

1. What is OSPF Cost?

2. Why OSPF Uses Cost?

3. How OSPF Cost is Calculated?

Cost = Reference Bandwidth / Interface Bandwidth\textbf{Cost = Reference Bandwidth / Interface Bandwidth}Cost = Reference Bandwidth / Interface Bandwidth

4. Common OSPF Cost Values (Default)

5. Manual Cost Configuration (Cisco)

6. Changing Reference Bandwidth (Important in Real Networks)

Key Points to Remember

$Bandwidth\textbf{Cost = Reference Bandwidth / Interface Bandwidth}$