AUTO-COST REFERENCE BANDWIDTH ON OSPF

In the cost of ospf  = 10^8 / bandwidth

At here 10^8 is equal to auto-cost reference bandwidth.

[Topology]

R1 [f0/0]   —- [f0/0] R2

R1(config-router)#no ?

auto-cost    Calculate OSPF interface cost according to bandwidth

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 100 <= [Here means 10^8]

Now, let us see how it works. As we had known that the default bandwidth of fast ethernet is 100 Mbps. Therefor, the cost of fast ethernet is 10^8/ 10^8 == 1

R1(config-if)#ip ospf 12 area 0

R1(config-if)#exit

R1(config)#do sh ip ospf int fa0/0

FastEthernet0/0 is up, line protocol is up

Internet Address 12.1.1.1/24, Area 0, Attached via Interface Enable

Process ID 12, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1 

R1(config-if)#bandwidth ?

<1-10000000> Bandwidth in kilobits

R1(config-if)#bandwidth 10000

R1(config-if)#do sh ip ospf int fa0/0

FastEthernet0/0 is up, line protocol is up

Internet Address 12.1.1.1/24, Area 0, Attached via Interface Enable

Process ID 12, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10  <=> [10^8/10000,000]