OSPF Reference Bandwidth

OSPF uses a simple formula to calculate the OSPF cost for an interface with this formula:

cost = reference bandwidth / interface bandwidth

The reference bandwidth is a value in Mbps that we can set ourselves. By default, this is 100Mbps on Cisco IOS routers. The interface bandwidth is something we can look up.

Let’s take a look at an example of how this works. I’ll use this router:

Cisco Router FastEthernet Serial Interface

The router above has two interfaces, a FastEthernet and a serial interface:

R1#show ip interface brief
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0       YES manual up                    up
Serial0/0             YES manual up                    up

Let’s enable OSPF on these interfaces:

R1(config)#router ospf 1
R1(config-router)#network area 0
R1(config-router)#network area 0

After enabling OSPF, we can check what the reference bandwidth is:

Router#show ip ospf | include Reference
 Reference bandwidth unit is 100 mbps

By default, this is 100 Mbps. Let’s see what cost values OSPF has calculated for our two interfaces:

Router#show interfaces FastEthernet 0/0 | include BW
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec
Router#show ip ospf interface FastEthernet 0/0 | include Cost
  Process ID 1, Router ID, Network Type BROADCAST, Cost: 1

The FastEthernet interface has a bandwidth of 100,000 kbps (100 Mbps), and the OSPF cost is 1. The formula to calculate the cost looks like this:

100.000 kbps reference bandwidth / 100.000 interface bandwidth = 1

What about the serial interface? Let’s find out:

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Forum Replies

  1. well explained.

  2. “It now has a cost of 1 which means that a Gigabit interface would end up with a cost of 1.”

    Did you mean 10? :wink:

  3. check this command its not working

    Router#show ip ospf | include Reference
     Reference bandwidth unit is 100 mbps

    this one is working

    Router#show ip protocols | include Reference

  4. Hey Rene,
    I’ve seen auto-cost reference-bandwidth as well as reference-bandwidth.
    Is there any difference between those two commands or are they just IOS version dependent?
    Thank you!

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